BIOLOGY 
RA 
G 


BIOLOGY 


BY 


GARY  N.  CALKINS,  PH.D. 

PROFESSOR  OF  PROTOZOOLOGY 
IN  COLUMBIA    UNIVERSITY 


SECOND  EDITION 
REVISED  AND  ENLARGED 


NEW  YORK 
HENRY  HOLT  AND  COMPANY 


•••       •''****•       •  *        •     •%  * 

»  •       *<-*•*••       «»««..*»*.• 


COPYRIGHT,  1914,  1917 

BY 
HENRY  HOLT  AND  COMPANY 


THE    MAPLE    PRESS    YO  R  K    PA 


PREFACE'"'-'-..  I-/'.,-". 

The  subject  matter  of  general  biology,  as  presented  in  current 
text  books,  is  variously  interpreted.  In  some  it  means  an  in- 
troduction to  the  essential  structures  and  vital  manifestations 
of  animals  and  plants.  In  others  it  means  the  discussion  of 
hypotheses  and  principles  of  biology.  In  others  it  becomes  an 
encyclopoedia  of  the  facts  of  physiology,  hygiene  and  ecology. 
With  the  first  method  the  course  is  based  largely  upon  labora- 
tory work  and  the  principles  are  illustrated  with  specific  types. 
The  second  and  third  methods  are  largely  didactic  and  are 
illustrated  by  examples  taken  at  random  from  the  entire  animal 
or  plant  kingdom. 

We  believe  thoroughly  in  the  type  and  laboratory  method  of 
instruction,  and  in  choosing  the  types  with  such  care  that  they 
serve  as  points  of  departure  for  various  lines  of  development  in 
subsequent  course  work.  The  present  work  is  based  upon  the 
excellent  course  outlined  in  Sedgwick  and  Wilson's  General 
Biology  which  occupies  so  prominent  a  place  in  the  teaching  of 
American  biology,  and  my  only  excuse  for  offering  another  to 
the  long  list  of  text  books  is  the  need,  which  we  have  felt  at 
Columbia,  of  a  work  along  similar  lines  to  cover  a  course  of 
about  thirty  class  exercises  and  as  many  laboratory  periods. 

The  book  is  planned  somewhat  differently  from  that  of 
Sedgwick  and  Wilson  partly  because  of  the  enlarged  scope, 
partly  because  of  the  excellent  general  introductory  courses 
offered  in  up-to-date  secondary  schools.  Emphasis  is  laid  at 
the  outset  on  cellular  activities,  especially  on  the  importance 
of  enzymes  in  metabolism  and  development,  while  animal 
differentiation  for  the  performance  of  primary  functions  of 
protoplasm  is  the  main  theme  of  the  entire  course.  In  the 
development  of  this  theme  organisms  of  one  cell,  organisms  of 
tissues,  and  organisms  of  organs  are  taken  up  in  succession. 

ill 


iv  PREFACE 

The  first  is  illustrated  by  yeasts,  bacteria,  and  protozoa;  the 
second  by  Hydra  and  the  coelenterates;  the  third  by  the  earth- 
worm. The  food  of  animals  and  the  sources  of  animal  energy 
are  treated  in  connection  with  Hydra  and  illustrated  by  the 
unicellular  plants  and  the  fern.  Further  differentiations  of 
organ  systems  are  illustrated  by  the  lobster  (or  crayfish),  and 
with  these  are  introduced  the  principles  of  homology  (through 
study  of  appendages)  and  of  morphological  adaptations.  This 
work  is  followed  by  a  short  study  of  physiological  adaptations  as 
illustrated  by  parasitism  and  by  some  of  the  phenomena  of  im- 
munity. As  the  general  theme  works  out  the  fundamental 
principles  of  evolution  are  developed  in  the  mind  of  the  student 
who  is  prepared  for  the  discussion  of  the  origin  and  perpetuation 
of  variations,  and  the  modern  principles  of  heredity  discussed 
in  the  last  chapter. 

For  permission  to  use  many  of  the  figures  I  am  indebted  to 
Professors  Sedgwick  and  Wilson;  to  the  Macmillan  Company 
for  cliches  of  figures  12,  24,  31,  88,  89;  to  Professor  Morgan 
and  the  Columbia  University  Press  for  cliches  of  figures  91,  93- 
101 ;  to  Lea  and  Febiger  for  cliches  of  figures  22,  27;  and  to  Miss 
Mabel  Hedge  for  the  original  drawings  reproduced  in  figures 
55 >  57>  69,  86;  finally  I  wish  to  express  my  grateful  appreciation 
to  Professor  J.  H.  McGregor  for  reading  the  manuscript  and  for 
many  helpful  suggestions  and  criticisms. 

GARY  N.  CALKINS 

COLUMBIA  UNIVERSITY 
NEW  YORK,  Sept.,  1914 


PREFACE  TO  THE  SECOND  EDITION 

In  the  present  edition,  although  there  is  no  change  in  the 
method  by  which  the  subject  of  Biology  is  developed,  there  are 
many  changes  in  the  text,  some  parts  being  condensed,  others 
elaborated,  in  the  interest  of  clearness.  Apart  from  verbal 
improvements  throughout  the  book,  the  most  important  altera- 
tions and  additions  have  been  made  in  connection  with  the  sub- 
jects of  fermentation  and  enzyme  activities;  the  significance  of 
conjugation;  plants,  the  food  of  animals;  photosynthesis;  circu- 
lation in  the  earthworm;  and  immunity.  Three  figures  in  the 
first  edition  (numbers  6,21,  and  39)  have  been  replaced  by  more 
instructive  illustrations,  and  in  all  cases  where  necessary,  the 
legends  have  been  amplified.  The  glossary,  which  was  intro- 
duced with  the  second  printing  of  the  first  edition,  is  consider- 
ably enlarged,  and  a  bibliography  added.  For  kindly  criticisms 
and  many  valuable  suggestions  in  connection  with  the  chapters 
dealing  with  plant  forms,  I  gratefully  acknowledge  here  my 
indebtedness  to  my  friend  Professor  H.  M.  Richards  of  Barnard 
College,  Columbia  University. 

G.  N.  C. 

April,  1917. 


CONTENTS 


PAGE 

INTRODUCTION I 

CHAPTER 

I.  LIVING  AND  LIFELESS  MATTER 6 

1.  The  Chemical  Composition 7 

2.  Metabolism  or  the  Power  of  Waste  and  Repair    ....  9 

3.  Growth  by  Intussusception 12 

4.  Reproduction  ....... 13 

5.  Power  of  Adaptation     . 15 

II.  PROTOPLASM  AND  THE  CELL,  AND  ORGANISMS  OF  ONE  CELL  ...  26 

A.  The  Organization  and  Vitality  of  Yeast  Cells 29 

B.  Bacteria 34 

C.  Enzymes,  Hormones  and  Vitamines  .    .    . 37 

III.  ORGANISMS  OF  ONE  CELL,  Continued.    .    .    .    .    .    .    ..  .    .    .    .  44 

A.  Amoeba  proteus '  .    .    .  44 

B.  Flagellated  Protozoa,  Chilomonas  and  Allied  Forms  .    .  53 

C.  A  Ciliated  Protozoon,  Paramecium  caudatum 60 

D.  Biological  Problems  associated  with  Protozoa      ....  65 

IV.  ORGANISMS  OF  TISSUES 76 

A.  Hydra  fusca  and  Hydra  viridis 82 

Histology ...,",:,'...  83 

Physiology 90 

Symbiosis 96 

Polymorphism 98 

B.  Summary 100 

V.  PLANTS,  THE  FOOD  OF  ANIMALS  AND  THE  SOURCES  OF  ANIMAL 

ENERGY  .    .    .   . 103 

A.  The  Food  of  Animals 103 

B.  Pleurococcus  pluviatilis  and  Sphaerella  lacustris.    .    .    .  106 

C.  Pteridium  aquilinum no 

Histology * 112 

General  physiology 1 1 8 

Reproduction 122 

VI.  ORGANS  AND  ORGAN  SYSTEMS 13° 

I.  General.-.    .    .    . 130 

II.  Structures  and  Functions  of  the  Earthworm,  Lumbri- 

cus  sp .'.'.• J31 

A.  Habits  and  Mode  of  Life 132 

B.  Regional  Differentiation 133 

,       C.  Internal  Structure ,    .  136 

vii 


viii  CONTENTS 

PAGE 

D.  Physiology  of  the  Digestive  System 139 

E.  Blood  Vascular  System 144 

F.  The  Excretory  System 146 

G.  The  Muscular  System 147 

H.  The  Nervous  System 149 

I.    The  Reproductive  System  ....'. 155 

J.    Reproduction.     Fertilization  and  Development.  158 

VII.    HOMOLOGY   AND   THE   BASIS  OF   CLASSIFICATION     .     .     ...     .     .  l62 

I.  The  American  Lobster,  Homarus  Americanus  .    .    .    .166 

Appendages  and  Serial  Homolcgy 167 

Digestive  System 172 

Blood  Vascular  System 173 

Excretory  System 177 

Muscular  System 177 

Nervous  System 179 

Sense  Organs 179 

Reproductive  System 181 

Development  and  Metamorphosis 182 

II.  General  Biological  Interest  of  the  Lobster 185 

III.  Insects 186 

VIII.  PARASITISM:  PHYSIOLOGICAL  ADAPTATION  .    .    . 190 

A.  The  Tape-worm  Taenia  sp 190 

B.  Animal  Associations 193 

C.  Adaptations  against  Parasites 195 

D.  The  Mechanism  of  Immunity 199 

IX.  THE  PERPETUATION  OF  ADAPTATIONS 203 

A.  Animal  Descent 204 

B.  Evolution 205 

C.  Conformity  to  type 207 

D.  Somatic  and  Germ  Plasm 208 

E.  The  Mendelian  Principles  of  Heredity 219 

A.  Heredity  of  One  Pair  of  Characters 219 

B.  Heredity  of  Two  Pairs  of  Characters 224 

C.  Heredity  of  Sex 225 

F.  The  Origin  of  Variations 232 


INTRODUCTION 

A  biologist  is  one  whose  subject  of  work  is,  or  has  been, 
living  matter.  The  subject  Biology,  however,  like  a  poly- 
hedron, has  numerous  faces,  each  one  more  or  less  circum- 
scribed and  independent  of  the  others.  Each  has  its  group 
of  followers  and  its  particular  name  and  may  be  looked  upon 
as  an  independent  science,  but  this  independence  is  quite 
superficial,  for  at  bottom  all  are  correlated  and  no  one  of 
them  is  more  entitled  to  the  name  Biology  than  any  other. 
It  is  customary  in  practice,  to  divide  the  biological  sciences 
into  two  groups  of  equal  value;  the  one  Zoology,  dealing 
with  animal  life,  past  and  present;  the  other,  Botany, 
dealing  with  plant  life.  They  may  also  be  divided  into 
two  unequal  groups,  Morphology  and  Physiology,  the  former 
dealing,  descriptively  for  the  most  part,  with  the  structures 
of  animals  and  plants,  the  latter  dealing,  experimentally 
for  the  most  part,  with  the  functions  or  vital  activities  of 
animals  and  plants.  This  latter  division,  however,  is  quite 
artificial  and  has  little  of  real  value. 

We  may  enumerate  and  correlate  the  biological  sciences 
in  some  such  manner  as  shown  in  the  accompanying  dia- 
gram (Fig.  i). 

All  of  the  biological  sciences  enumerated  here  may  have 
as  the  subject  matter  either  animals  or  plants.  Thus  there 
is  a  plant  and  animal  physiology,  plant  and  animal  anat- 
omy or  morphology,  etc.  Furthermore,  in  addition  to 
the  main  sciences  there  are  numerous  subsidiary  branches 
which  deal  with  special  groups,  such  for  example  as  Bac- 
teriology, Algology,  Entomology,  Protozoology,  etc.,  most 
of  which  are  specialized  subdivisions  of  one  or  more  of  the 
sciences  given  above.  Of  these,  Anatomy  deals  with  the 

1 


2  INTRODUCTION 

general  structures  of  the  body  and  is  purely  descriptive  in 
character.  It  has  been  most  extensively  developed  in  con- 
nection with  the  human  organism  where  an  accurate  knowl- 
edge of  the  muscles,  arteries,  nerves  and  various  organs 
of  the  body  is  of  the  greatest  importance  to  medical  men. 

ANIMAL 


PIANT 

FIG.   i. — Diagram  to  show  the  relation  of  General  Biology  to    the  biological 

sciences. 


Comparative  anatomy  has  also  been  developed  as  an  aid 
primarily,  to  the  understanding  of  the  various  organs  in 
man.  Embryology,  too,  is  an  important  adjunct  to  anat- 
omy for  it  deals  with  the  development  of  organs  in  the  indi- 
vidual. It  also  has  a  broader  purpose  in  general  biology  and 


BIOLOGICAL  SCIENCES  3 

has  grown  into  a  science  quite  apart  from  any  particular  bear- 
ing on  human  affairs.  Cytology  is  the  science  dealing  with 
the  ultimate  units  of  structure  of  living  things — cells — and 
has  both  physiological  and  morphological  sides,  while,  in 
connection  with  the  germ  cells,  it  comes  in  intimate  con- 
tact with  the  fundamental  problems  of  general  biology.  His- 
tology deals  with  the  aggregates  of  cells  in  tissues,  and  this 
branch,  again,  is  mainly  important  to  students  of  medicine. 
Pathology  deals  with  abnormal  structures  and  functions 
of  animals  and  plants,  and  is  essentially  a  science  of  dis- 
ease. Palaeontology  deals  exclusively  with  past  life  on  the 
earth  as  revealed  by  fossil  forms  of  plants  and  animals.  Taxo- 
nomy is  the  science  of  classification  and  is  dependent  upon 
anatomy,  embryology  and  ecology. 

The  sciences  enumerated  above  are  mainly  morphological 
or  based  upon  the  structures  of  living  things,  that  is  upon 
the  mechanisms  employed  in  the  performance  of  vital  ac- 
tivities. The  remaining  five  branches  are  essentially  physiolog- 
ical or  based  upon  the  actions  of  the  living  mechanisms. 
In  this  group  the  science  of  physiology  is  the  oldest  and  the 
best  established,  dealing,  as  it  does,  with  the  fundamental 
activities  of  digestion,  assimilation,  respiration,  excretion, 
secretion,  nerve  response  and  reproduction.  Experimental 
biology,  ecology  and  genetics  are  more  recent  and  have  been 
developed  in  connection  with  the  attempts  to  throw  light  upon 
the  fundamental  biological  principles  of  growth,  differentiation, 
inheritance,  variability  and  organic  relationships.  Ecology 
deals  with  animals  and  plants  as  affected  by  environment, 
distribution  on  the  earth's  surface,  and  the  like.  Genetics 
deals  with  the  different  phases  of  the  problems  of  heredity. 
Neurology,  finally,  including  psychology,  is  a  science  deal- 
ing with  the  nervous  system  and  with  the  attributes  of  the 
brain,  while  a  corresponding  science  deals  with  the  phenomena 
of  sensation,  irritability,  etc.,  in  plants. 

Other  sciences  such  as  Sociology,  Anthropology,  Political 
Economy,  etc.,  dealing  with  man,  have  a  certain  claim  to 
relationship  with  the  biological  sciences,  but,  except  in  a  very 


4  INTRODUCTION 

general  way,  they  are  not  usually  included  with  this  group  of 
sciences. 

The  diagram  (Fig.  i)  also  illustrates  in  a  general  way,  the 
manner  in  which  General  Biology  is  related  to  the  various 
biological  sciences.  It  must  not  be  understood  that  the 
several  divisions  which  we  recognize  today  have  grown  out 
of  any  maternal  science  of  biology.  On  the  contrary  the 
principles  of  General  Biology  represent  contributions  from 
all  the  related  sciences,  and  these  contributions,  for  the 
most  part,  are  fundamental  or  basic  for  the  special 
branches  involved.  Physiology,  perhaps  more  than  any  other 
branch,  is  intimately  connected  with  General  Biology;  indeed 
for  a  long  early  period  General  Biology  and  Physiology  were 
indistinguishable.  Anatomy  also  was,  and  is  yet,  intimately 
correlated  with  Physiology,  and  from  these  two  main  trunks 
the  secondary  branches  have  developed  into  special  fields 
of  research.  One  great  stimulus  for  this  development  was 
the  doctrine  of  evolution  which  has  been  mainly  responsible 
for  the  distinctly  modern  sciences  of  Ecology,  Experimental 
Biology  and  Genetics. 

What  then  is  the  subject  matter  of  General  Biology?  Many 
naturalists  refuse  to  recognize  it  or  give  it  a  place  in  their 
teachings,  while  the  majority  of  Universities  have  substituted 
departments  of  Zoology  and  Botany  and  Physiology  for  the 
erstwhile  department  of  Biology. 

General  Biology  deals  with  the  fundamental  principles 
of  living  matter;  specifically,  first,  with  protoplasm  and  with 
the  manifestations  of  vitality;  second,  with  metabolism,  or 
the  vital  processes  of  waste  and  repair;  third,  with  the  food 
of  animals  and  plants  and  with  the  ultimate  sources  and  trans- 
formations of  energy;  fourth,  with  the  fundamental  struc- 
tures of  living  things  and  with  the  evolution  of  organic  struc- 
tures; fifth,  with  the  inter-relations  of  animals,  plants,  and 
intermediate  organisms;  sixth,  with  the  phenomena  of  vitality, 
adolescence,  age  and  senescence,  fertilization,  reproduction, 
and  heredity;  and,  seventh,  with  species  and  the  factors  of 
organic  evolution.  Obviously  if  we  were  to  study  any  one 


GENERAL  BIOLOGY  5 

of  these  topics  to  the  limits  of  our  knowledge  concerning 
it,  we  would  compass  the  entire  realm  of  the  biological 
sciences,  so  intimately  is  general  biology  related  to  them 
all.  This,  however,  is  just  what  General  Biology  should  not 
do;  it  should,  rather,  provide  a  foundation  suitable  for  the 
further  study  of  any  one  or  all  of  the  many  branches  of 
biological  science. 


CHAPTER  I 
LIVING  AND  LIFELESS  MATTER 

THE  biological  sciences  all  agree  in  their  fundamental 
subject  matter,  i.e.,  they  all  deal  with  things  that  are,  or  have 
been,  alive.  In  this  one  fundamental  fact  they  differ  from 
the  physical  sciences.  The  boundaries  between  the  bio- 
logical and  the  physical  sciences  are  very  indefinite,  however, 
and  investigations  into  the  nature  of  life,  or  indeed  of  any 
of  its  manifestations,  would  be  of  a  very  superficial  type 
were  not  the  physical  sciences  involved.  Biological  and 
physiological  chemistry,  as  branches  of  the  science  of 
physiology,  are  really  branches  of  chemistry,  but  their  subject 
matter  is  material  that  has  been  living,  or  has  been  derived 
from  living  things. 

What  then  is  living  matter  as  distinguished  from  non- 
living matter? 

All  animals  and  all  plants  are  made  up  of  a  fundamental 
living  substance,  together  with  derivatives  from  this  sub- 
stance, to  which  the  name  Protoplasm  was  given  by  Pur- 
kinje  in  1840.  The  term  protoplasm  cannot  be  accurately 
defined  because  it  represents  a  conception  rather  than  a  defi- 
nite thing,  there  being  almost  as  many  protoplasms  as  there 
are  animals  and  plants.  The  term  should  be  used  much  as 
we  use  the  terms  animal  and  plant,  which  refer  to  no  special 
animal  or  plant,  and  it  cannot  be  described  any  more  accurately 
than  can  these  concepts.  Huxley  has  called  it  the  "Physi- 
cal Basis  of  Life,"  and  the  physiologist  duBois  Reymond 
described  it  as  the  "Agent  of  Vital  Manifestations."  It 
is  obvious  that  neither  of  these  definitions  would  enable  us 
to  recognize  living  substance.  The  nearest  approach  to  a 
description  of  protoplasm  is  to  describe  the  properties  which 
protoplasms  have  in  common. 

6 


PROTOPLASM  7 

The  most  essential  characteristic  of  this  group  of  similar 
substances,  which  we  designate  Protoplasm,  is  that  they  lose 
their  characteristics  with  life,  that  is  to  say  they  are  no  longer 
protoplasm  when  life  is  gone.  The  properties  however  which 
living  matter  possesses  alone  of  all  things,  are  derived  from 
characteristics  of  protoplasm  both  in  the  living  state  and  from 
its  material  basis  when  life  is  gone.  These  properties  as 
usually  given  are  (i)  the  chemical  composition;  (2)  the  power  of 
waste  and  repair;  (3)  the  power  of  growth  by  intussusception; 
(4)  the  power  of  fertilization  and  reproduction,  and  (5)  the 
power  of  adaptation. 

i.  THE  CHEMICAL  COMPOSITION 

Chemical  composition,  naturally,  is  one  of  the  properties 
of  protoplasm  which  can  be  obtained  only  after  life  is  gone, 
analytical  processes  invariably  killing  it.  Nevertheless  there 
is  no  loss  of  weight  after  death,  so  presumably  the  same  chem- 
ical elements  are  present.  Analyzed  in  bulk,  material  that 
has  been  living  is  known  to  contain  Carbon,  Hydrogen,  Nitro- 
gen, Oxygen,  Sulphur,  Phosphorus,  Fluorine,  Chlorine,  Silica 
and  metals  Na  (sodium),  K  (potassium),  Ca  (calcium),  Mg 
(magnesium),  Fe  (iron),  etc.  The  chemical  composition  is 
not  easy  to  determine  because  protoplasm  is  not  a  homogene- 
ous substance  but  a  mixture  of  different  substances;  the 
elements  given  above  are  combined  in  a  great  variety  of  ways 
of  which  more  or  less  definite  compounds  called  albuminous 
compounds,  or  proteins,  albuminoids,  and  nucleo-proteins 
(all  of  which  are  grouped  together  under  the  general  term  pro- 
teins) are  universally  present.  Hoppe-Seyler  in  1871  analyzing 
pus  cells  free  from  the  surrounding  fluids,  found  the  following 
percentages  of  substances: 

Nuclein 34. 257  per  cent. 

Insoluble  substances 20. 566  per  cent. 

Lecithin  and  fat 14 . 383  per  cent. 

Cholesterin 7 . 40    per  cent. 

Cerebrin 5 . 199  per  cent. 

Undetermined  albuminoids 13 . 762  per  cent. 

Extractives 4-433  per  cent. 


8 


LIVING  AND  LIFELESS  MATTER 


In  the  ash  he  found  sodium,  potassium,  iron,  magnesium, 
calcium,  phosphoric  acid  and  chlorine.  Since  then  a  great 
variety  of  different  substances  have  been  obtained  from  cells 
and  tissues  of  different  living  things  some  of  which  are  given 
in  the  following  partial  classification  of  the  proteins. 

CLASSIFICATION  OF  THE  PROTEINS 


Albumins 

Globulins 

Uric  acid 

A.  Simple    Pro- 

Glutelins 

Xanthine 

teins    (albumi-  ' 

Prolamines 

i-methylxanthine 

nous  bodies) 

Albuminoids 

Heteroxanthine 

Histones 

Theophylline 

Protamines 

C  Nucleoproteins 

.                   Glycoproteins 
B.  Conjugated     1       '     f            . 
Proteins              Phosphoprotems 

Nucleic  acids 
Purine  bases     • 
Pyrimidine 
bases 

Paraxanthine 
Theobromine 
Caffeine 
Hypoxanthine 
Guanine 

J    IvJLClllo                         I     TT                        ii« 

Haemoglobins 

Epiguanine 

I  Lecithoproteins 

Adenine 

Episarkine 

Carnine 

C.  Derived 
Proteins 


f  Primary 

derivatives 


|  Secondary 
derivatives 


(  Proteans 
J  Metaproteins 
I  Coagulated 
I      proteins 
(  Proteoses 
\  Peptones 
I  Peptides 

The  above  list  does  not  give  anything  like  a  complete  enu- 
meration of  the  chemical  compounds  which  make  up  proto- 
plasm. Most  of  the  above  are  merely  compounds  of  C,  H, 
N,  O,  and  P,  and  vary  in  the  relative  percentages  of  the  different 
elements  in  combination.  In  the  list  only  the  derivatives  of 
the  purine  bases  are  given,  but  each  of  the  others  includes 
a  similar  list  of  substances.  Add  to  these  many  compounds 
the  various  combinations  of  carbohydrates  and  fats,  and  of  the 
minerals  Ca,  Na,  Fe,  Mg,  etc.,  and  some  faint  conception  may 
be  gained  of  the  enormous  number  of  chemical  bodies  to  be 
found  in  protoplasm. 


PROPERTIES  OF  PROTOPLASM          9 

Chemically,  living  matter  may  be  summarized  as  consist- 
ing of  proteins,  fats,  carbohydrates  and  salts,  the  latter  play- 
ing some  important  part  in  the  vital  processes  although,  since 
they  all  do  not  appear  in  all  types  of  protoplasm,  each  cannot 
be  regarded  as  a  sine  qua  non  of  living  matter.  The  absolutely 
essential  elements  are  carbon,  hydrogen,  nitrogen,  oxygen,  and 
phosphorus  which  enter  into  the  composition  of  pure  nucleinic 
acid  and  form  the  basis  of  all  protoplasm. 

While  chemical  analysis  gives  an  idea  of  the  kinds  of  ele- 
ments entering  into  the  composition  of  protoplasm  after  death, 
it  allows  no  conception  of  the  numbers  of  chemical  bodies 
that  are  continually  being  formed  during  life,  and  still  less 
conception  of  the  nature  of  the  vital  chemical  processes. 
It  is  generally  agreed  that  pure,  ash-free  proteins  are  really 
inert  and  lifeless  and  that  salts  or  electrolytes,  either  organic 
or  inorganic,  are  necessary  for  the  vital  activities. 

Chemical  composition,  therefore,  does  not  carry  us  very 
deeply  into  the  mysteries  of  protoplasmic  composition,  nor 
does  it  give  any  clue  to  the  nature  of  the  vital  processes. 
It  shows,  however,  what  chemical  elements  are  essent-"  il  for 
continued  life,  i.e.,  what  elements  are  necessary  to  pr'.  vide  for 
in  the  food,  for  all  living  things  are  constantly  using  up  these 
substances  in  vital  activities  and  replacing  them  from  the  food 
materials  selected  from  the  environment.  This  dual  process 
of  waste  and  repair,  met  with  nowhere  save  in  living  matter, 
is  a  secondary  fundamental  property  of  living  things  and  is 
generally  spoken  of  under  the  heading  metabolism. 

2.  METABOLISM    OR    THE    POWER    OF    WASTE    AND    REPAIR 

A  very  good  idea  of  the  effects  of  continued  protoplasmic 
activity  in  the  absence  of  food  may  be  obtained  by  keeping  some 
minute  a^nimal,  for  example  a  protozoon  like  Paramecium,  in  a 
sterile  medium  for  a  few  days.  Paramecium  is  a  microscopic 
water-dwelling  animal  to  be  found  in  any  stagnant  ditch  or 
pond.  Ordinarily  it  swims  about  actively  by  means  of  minute 
motile  organs  termed  cilia  and  takes  in  as  food  still  more  minute 


10 


LIVING  AND  LIFELESS  MATTER 


bacteria  with  the  constant  current  entering  the  mouth.  If  it  is 
transferred  to  a  sterile  medium  it  gets  little  or  no  food  and  the 
protoplasm  begins  to  waste  away.  The  first  effect  of  this  un- 
compensated  waste  is  the  appearance  of  spaces  or  vacuoles, 
and  after  some  time  in  this  skeleton-like  condition  the  organism 
dies  (Fig.  2).  In  other  cases  the  effect  may  be  shown  by  a 


• 


FIG.  2.  FIG.  3. 

FIG.  2. — Effect  of  starvation  in  Paramecium  caudatum.  Photographs  (same 
magnification)  of  normal  (at  right)  and  starved  individuals. 

FIG.  3. — Effects  of  starvation  in  Dileptus  gigas.  Photographs  (same  magnifi- 
cation) of  preparations  of  normal  individual  and  individuals  starved  ten  and 
twenty-one  days  respectively.  All  sister  cells. 


constantly  diminishing  size;  Fig.  3  represents  a  normal  speci- 
men of  the  protozoon  Dileptus  gigas  and  sister  organisms 
starved  for  ten  and  twenty-one  days. 

What  happens  in  these  small  living  things  finds  a  rough  anal- 
ogy in  a  coal  fire.  The  coal,  made  up  of  carbon  and  inorganic 
matter,  is  rapidly  oxidized,  and  energy  in  the  form  of  light  and 


PROPERTIES  OF  PROTOPLASM  11 

heat  is  liberated.  The  potential  energy  thus  changed  into  light 
and  heat  was  stored  up  in  the  coal,  ages  ago  when  it  was  a  part 
of  the  earth's  vegetation.  This  process  of  physical  combustion 
is  brought  about  by  the  union  of  oxygen  (oxidation)  with  vari- 
ous elements  in  the  coal.  Smoke,  carbon  dioxide  (C02),  water 
(H2Q),  and  an  incombustible  residue  (ash)  are  formed,  while 
kinetic  energy  is  given  off  in  the  form  of  light  and  heat.  Here 
then,  with  oxidation  is  a  change  from  potential,  or  stored,  to 
kinetic  or  free  energy,  while  an  organic  material  with  definite 
properties  is  changed  at  the  same  time  into  CO2,  H2O,  free 
carbon  and  a  useless  residue. 

The  famous  French  chemists,  Lavoisier  and  Laplace,  in  1780, 
were  the  first  to  show  that  animal  heat,  like  that  from  fire,  is 
produced  by  combustion  involving  the  consumption  of  oxygen 
and  the  liberation  of  CO2,  and  they  found  that  practically  the 
same  amount  of  heat  was  produced  and  the  same  amount  of 
CO2  was  liberated  by  a  living  Guinea  pig  and  by  a  burning 
candle.  Later,  it  was  discovered  that  another  product  occurs 
in  the  living  animal,  viz.  urea. 

The  actively  moving,  eating,  digesting  and  excreting  Para- 
mecium  gets  the  energy  for  its  many  vital  processes  through 
the  oxidation  of  substances  contained  in  its  protoplasmic  make- 
up. As  in  the  combustion  of  coal,  CO2  and  H2O  are  formed  and 
liberated  while  an  incombustible  residue,  termed  urea,  is  analo- 
gous to  ashes  in  physical  combustion.  The  energy  for  move- 
ments and  for  carrying  on  the  many  physiological  activities  of 
the  organism  is  derived  from  the  chemical  energy  contained  in 
the  complex  molecules  forming  the  basis  of  all  protoplasm.  The 
continued  activity  of  Paramecium  without  a  new  supply  of  fuel 
(food)  results  in  the  burning  out  of  the  protoplasmic  substance 
as  shown  by  the  vacuolization  of  the  body,  final  exhaustion  of 
the  available  elements  for  combustion,  and  must  result  in  death 
(Figs.  2  and  3).  Similar  processes  take  place  in  all  animals  and 
plants;  C02,  H20,  and  urea  or  equivalent  are  formed  and  ex- 
creted in  one  way  or  another,  while  many  of  the  complexities 
in  structure  of  the  higher  animals  are  due  to  the  elaboration  of 
organs  for  the  disposal  of  such  waste  products. 


12  LIVING  AND  LIFELESS  MATTER 

To  continue  the  analogy  a  bit  further.  New  fuel  is  needed 
to  maintain  a  fire,  so  new  food  is  needed  to  prevent  death 
through  continued  waste  and  to  provide  energy  for  continued 
activity.  The  new  coal  must  first  undergo  a  certain  amount  of 
preparation  before  it  undergoes  oxidation;  it  must  be  broken 
into  small  pieces,  and  must  be  raised  to  a  certain  temperature 
before  active  combustion  takes  place.  Similarly,  food  con- 
sisting usually  of  lifeless  proteins,  carbohydrates  and  fats 
derived  from  other  animals  or  plants,  must  be  disintegrated 
and  prepared  for  assimilation,  and  this  finely  divided  food 
material  is  then  distributed  to  all  parts  of  the  organism.  The 
process  of  thus  preparing  the  food  is  called  digestion,  and  is 
mainly  a  process  of  hydrolysis  of  food  materials.  In  biology 
the  two  processes  of  waste  and  repair  are  usually  considered 
together  under  the  term  Metabolism,  destructive  metabolism 
called  katabolism  being  the  sum  of  processes  concerned  with 
the  breaking  down  or  combustion  of  protoplasmic  substances, 
and  constructive  metabolism,  called  anabolism,  being  the  sum 
of  processes  having  to  do  with  repair  and  growth.  Just  how  the 
finely  divided  food  particles  are  added  to  the  protoplasmic 
molecules  is  unknown,  but  it  is  certain  that  the  addition  takes 
place  uniformly  and  in  all  parts  of  the  organism,  and  that  by 
such  uniform  additions  of  new  materials  growth  of  the  organism 
takes  place.  From  its  mode  of  addition  we  have  the  third 
property  of  living  matter: 

3.  GROWTH  BY  INTUSSUSCEPTION 

This  is  distinguished  from  growth  by  accretion  as  seen  in  the 
enlargement  of  a  crystal,  for  example,  where  new  particles  are 
added  to  the  outside  of  the  existing  structure.  Obviously  such 
growth  or  increase  in  amount  of  protoplasm  together  with  its 
differentiation,  can  take  place  only  when  the  waste  or  combus- 
tion of  protoplasmic  substances  is  less  than  the  new  materials 
added.  When  the  constructive  processes  exceed  the  destruc- 
tive, more  material  is  added  to  the  protoplasm  than  is  lost 
by  waste,  and  growth  results.  This  growth  continues  until  a 


PROPERTIES  OF  PROTOPLASM 


13 


certain  limit  of  size  is  reached,  every  animal  and  every  plant 
having  such  a  limit  to  size,  and  to  form  as  well,  and  when  this 
limit  is  reached  many  of  the  physiological  activities  are  directed 
toward  reproduction  of  the  race  or  of  kind.  Such  a  stage  is 
spoken  of  as  the  period  of  maturity,  and  it  varies  according  to 
the  total  length  of  life  of  the  organism.  This  period  of  maturity 
is  preceded  by  a  period  of  active  growth  when  constructive 
processes  are  far  in  excess  of  the  destructive,  and  is  called  the 
period  of  youth.  Succeeding  this  comes 
a  period,  adolescence,  of  greater  equilib- 
rium between  constructive  and  de- 
structive processes,  and  with  this  comes 
a  period  of  maturity  with  the  power  to 
reproduce  the  species.  This  power 
marks  a  fourth  property  of  protoplasm : 

4.  REPRODUCTION 

Reproduction  of  kind  is  a  phenom- 
enon exclusively  confined  to  living 
things  but  the  manner  of  reproduction 
varies  in  different  cases.  In  some  cases 
the  living  organisms  divide  through  the 

middle  to  form  two  similar  halves,  each      ,, 

,          FIG.  4. — Division  of  Eu- 

of  which  forms  a  new  organism.  This,  plotes  patella.  Photograph 
the  most  simple  method  of  reproduc-  from  a  PreParation- 
tion,  is  called  simple  division  or  binary  fission  (Fig.  4) .  Again, 
minute  bits  of  the  organism  may  be  pinched  off  the  periphery 
of  the  parent,  and  these  grow  into  organisms  similar  to  the  par- 
ent (Fig.  5).  This  method  is  termed  budding  or  gemmation, 
and  the  buds  thus  formed  may  be  single  or  multiple  in  number, 
as  in  Hydra.  Sometimes  the  entire  substance  of  the  parent 
organism  breaks  into  minute  reproductive  bodies  to  which 
the  term  spores  is  applied,  the  process  being  known  as  repro- 
duction by  spore-formation  or  sporulation  (Fig.  6).  All  of 
the  above  methods  of  reproduction  are  limited  to  the  lower 
types  of  organisms,  while  the  higher  types  reproduce  by  proc- 


14 


LIVING  AND  LIFELESS  MATTER 


FIG.  5. — Five  specimens  of  Hydra  fusca  on  water  plant.     One  individual  in  proc- 
ess of  budding.     (From  a  drawing  by  S.  F.  Denton  made  for  E.  B.  Wilson.) 


PROPERTIES  OF  PROTOPLASM 


15 


esses  involving  sex  differentiation,  and  are  called  sexual  repro- 
duction. These  consist,  both  in  animals  and  plants,  in  the  fer- 
tilization of  an  egg  derived  from  a  female  organism,  by  a 
spermatozoon  derived  from  a  male,  and  after  the  union  of  egg 
and  spermatozoon  an  embryo  is  produced  which  grows  through 
many  different  stages  in  development  (ontogeny)  to  an  adult 
organism  similar  to  the  parent  form.  In  some  cases  the  egg 


FIG.  6. — Asexual  speculation  in  malaria  organisms.  A,  parasite  of  tertian 
malaria  (Plasmodium  vivax)  in  human  blood  corpuscle;  B,  multiple  division 
(schizogony)  of  same;  C,  multiple  division  of  the  organism  causing  quartan 
malaria  (Plasmodium  malaria);  c,  blood  corpuscles;  m,  melanin  granules 
formed  by  the  parasites  and  liberated  into  the  blood  as  a  toxin  during  sporula- 
tion;  n,  nuclei  of  parasite  and  progeny  (merozoites) ;  p,  cell-body  of  parasite;  v, 
vacuole.  From  preparations. 

proceeds  to  develop  without  processes  of  fertilization,  such  a 
method  of  reproduction  being  known  as  parthenogenesis,  a 
result  which  may  be  brought  about  in  some  cases,  artificially,  by 
the  use  of  salts. 


5.  POWER  OF  ADAPTATION 

A  fifth  property  possessed  by  protoplasm  is  the  capacity  to 
vary  under  changed  conditions  of  the  environment.  It  is  by 
reason  of  this  power  that  the  myriads  of  animal  and  plant  forms 
exist  today  as  distinct  species.  Such  variations,  due  perhaps 
to  environmental  differences,  perhaps  to  mutations  or  sudden 
and  unexplained  appearance,  are  rarely  observed  in  the  making, 
but  the  result  of  the  change  or  changes  is  spoken  of  as  an  adapta- 
tion. Such  adaptations  may  be  in  structure  or  in  function,  and 


16  LIVING  AND  LIFELESS  MATTER 

are  accordingly  either  morphological  or  physiological.  (See 
Chapter  IX.) 

These  properties,  chemical  composition,  power  of  waste  and 
repair,  growth  by  intussusception,  power  of  reproduction  and 
adaptability  are  primary  attributes  of  protoplasm,  and  serve  to 
distinguish  living  from  all  other  kinds  of  matter.  It  does  not 
follow,  however,  that  non-living  things  do  not  manifest  one  or 
more  of  these  phenomena.  Thus  the  chemical  composition, 
as  we  have  seen,  is  that  of  lifeless  protein,  while  growth  by 
intussusception  may  be  said  to  take  place  whenever  a  solid 
crystalloid  is  dissolved  in  a  liquid.  Furthermore  these  proper- 
ties are  of  such  a  nature  that  if  we  were  dependent  upon  them  it 
would  be  difficult  in  some  cases  to  tell  whether  an  organism  is 
alive  or  not.  To  determine  its  chemical  composition  it  would 
have  to  be  killed;  its  growth,  waste  and  repair  could  not  be 
easily  observed,  while  only  a  fortunate  chance  would  reveal  its 
reproduction.  It  is  possible,  however,  to  determine  by  certain 
characteristics  of  protoplasm  whether  a  given  thing  is  living 
matter  or  not  without  the  necessity  of  ascertaining  its  prop- 
erties, and  these  we  speak  of  as  the  evidences  or  manifestations 
of  vitality. 

These  are  usually  included  under  the  heads  of  appearance, 
form  and  movement. 

PROTOPLASMIC  APPEARANCE. — Under  a  microscope,  proto- 
plasm has  a  characteristic  and  recognizable  appearance  not 
easy  to  describe.  If  seen  with  a  low  magnification  it  appears 
like  a  transparent,  colorless,  somewhat  glass-like,  semi-fluid 
substance  usually  with  numerous  granules  of  variable  size  and 
with  many  clear  spaces  or  vacuoles.  It  is  always  refringent 
and  never  mixes  with  the  surrounding  water.  It  is  viscous; 
has  a  high  power  of  cohesion  and  readily  absorbs  substances 
by  osmosis  from  the  surrounding  medium  and  gives  off  sub- 
stances, also  by  osmosis,  to  the  surrounding  medium.  It  is, 
therefore,  permeable  in  respect  to  some  substances.  If  seen 
under  a  high  magnification  the  appearance  differs  with  the  ob- 
ject. In  some  cases  there  is  a  more  or  less  definite  reticulum 
or  network  enclosing  a  more  fluid  substance;  in  other  cases  the 


MANIFESTATIONS  OF  VITALITY  17 

protoplasm  appears  like  an  aggregate  of  bubbles  in  a  frothy  soap 
suds,  with  the  walls  of  the  bubbles  or,  in  protoplasm,  alveoli, 
relatively  dense  and  the  intra-alveolar  substance  relatively 
more  fluid;  in  still  other  cases  the  more  refringent  parts  are  in 
the  form  of  minute  rods  or  nbrillae  surrounded  by  a  more  iluid 
matrix.  In  all  forms  assumed  by  protoplasm,  there  are  in- 
variably fine  granules,  called  microsomes,  scattered  throughout. 

These  different  types  of  protoplasmic  structure  have  given 
rise  to  different  theories  as  to  the  physical  make-up  of  proto- 
plasm, and  the  adherents  of  each  theory  hold  that  all  other 
appearances  are  only  modifications  of  the  structures  which  they 
believe  fundamental.  Thus  we  find  biologists  who  hold  to  the 
"reticular"  theory,  others  who  hold  the  "  alveolar "  theory, 
others  again  who  adhere  to  the  "fibrillar"  theory,  and  still 
others  who  maintain  that  all  apparent  structures  are  secondary 
and  unimportant  and  that  the  only  vital  elements  in  the  physi- 
cal make-up  are  the  granules  or  microsomes.  Whatever  may 
be  the  outcome  of  disputes  over  the  relationship  of  the  differ- 
ent appearances  the  fact  remains  that  protoplasm  consists  of 
an  aggregate  of  fluid-like  substances  of  different  densities  which 
may  assume  a  variety  of  configurations. 

FORM. — These  appearances,  even  if  uniform,  could  not  be 
relied  upon  as  a  sure  manifestation  of  vitality.  Lifeless  protein, 
albumen,  and  even  emulsions  of  oil,  water  and  salt,  give  similar 
appearances  so  that  other  manifestations  must  be  taken  con- 
jointly. Appearance  combined  with  form  gives  fairly  definite 
evidence  of  life.  Form,  however,  is  closely  connected  with  the 
configuration  of  morphological  units  of  protoplasmic  structures. 
With  the  exception  of  a  small  number  of  amorphous  living 
things,  all  types  of  animals  and  plants  have  a  definite  and  recog- 
nizable form.  No  living  thing  consists  of  a  homogeneous  sheet 
or  column  or  ball  of  semi-fluid  protoplasm,  but  in  all  higher 
types  the  protoplasm  is  divided  among  myriads  of  very  tiny 
units  called  CELLS  which  may  become  differentiated  in  the  great- 
est variety  of  ways.  The  few  amorphous  types  of  living  things 
consist,  as  a  rule,  of  but  one  single  cell  (e.g.,  Amoeba,  Fig.  10). 
In  life  the  individual  cells  of  an  animal  or  plant  cannot  be  readily 


18 


LIVING  AND  LIFELESS  MATTER 


made  out;  some  of  them  are  glandular  in  function,  others  are 
muscular,  others  sensory,  supporting,  reproductive,  etc.,  the 
form  of  the  organism  being  due  largely  to  the  supporting  cells 
and  products.  With  the  exception  of  the  amorphous  types  all 
animals,  even  the  unicellular  ones,  have  fairly  definite  axes  of 
symmetry.  Some,  the  globular  ones,  are  homaxonic  or  similar 
in  structure  in  all  planes  passing  through  the  center ;  others  are 
monaxonic,  having  but  one  axis  of  symmetry,  with  the  mouth 
at,  or  near,  one  extremity  termed  anterior,  and  the  tail  with 
vent  at  the  opposite  end  termed  posterior.  In  such  forms  the 
mouth  side  is  ventral,  the  opposite  or  aboral  side  is  dorsal.  Still 
other  types  are  polyaxonic  and,  like  Hydra,  may  be  divided  by 
innumerable  vertical  planes  into  symmetrical  halves. 


FIG.  7. — Lifeless  matter  in  living  cells,  c,  Groups  of  crystals  of  calcium  oxal- 
ate;  i.e.,  intercellular  space;  n,  nucleus;  p,  cytoplasm;  s,  starch  granules;  /,  fat 
drops.  (From  Sedgwick  and  Wilson.) 

The  form  of  an  animal  or  plant,  dependent  largely  upon  the 
presence  and  activity  of  the  supporting  cells,  is  very  often  due 
to  the  presence  of  lifeless  matter  within  or  around  those  cells 
and  created  by  them.  In  the  majority  of  cases  the  form  is 
retained  for  a  longer  or  shorter  time  after  the  living  protoplasm 
has  died,  hence  form  alone  would  be  insufficient  evidence  of 
living  matter.  Many  lifeless  things,  such  as  crystals  and  life- 
less products  of  vital  activity,  may  also  have  definite  forms, 
hence  neither  form  alone  nor  form  with  appearance  would 
give  a  complete  manifestation  of  vitality.  Living  and  lifeless 


MANIFESTATIONS  OF  VITALITY 


19 


matter  often  go  together  to  make  up  the  form  of  an  organism, 
the  lifeless  matter  being  laid  down  either  within  the  cells  or 
around  the  cells.  Many  products  of  waste  metabolism  are 
thus  stored  up  in  living  cells,  perhaps  to  serve  some  useful  pur- 
pose in  the  functional  activities,  or  to  await  some  means  of 
disposal.  Crystals  are  often  found  in  cells  (Fig.  7),  and  all 
vegetable  and  some  animal  forms  make  and  store  up  starch 
grains,  sometimes,  as  in  the  case  of  the  potato,  in  great 
quantities. 


FIG.  8. — Lifeless  matter  around  living  cells,  c,  Cartilage  cells  surrounded  by 
lifeless  matrix,  m;  and  branching  bone  cells  in  the  lifeless  bony  matrix  (at  right). 
(From  Sedgwick  and  Wilson.) 

Fat  also  is  stored  up  frequently  in  cells,  and,  like  starch,  be- 
comes a  reserve  store  of  nutriment.  Again,  living  cells  secrete 
about  themselves  different  kinds  of  lifeless  matter  for  purposes 
of  support,  protection,  defense,  etc.  Cartilage  cells  become 
surrounded  by  a  lifeless  matrix  of  hard  resistant  cartilage,  some 
kinds  of  which  become  replaced  by  deposition  of  calcium  phos- 
phate and  thus  become  bone  with  which  the  living  cells  of  the 
former  cartilage  are  entirely  displaced  (Fig.  8).  Similarly 
living  blood  elements  float  in  a  lifeless  matrix  of  fluid  plasm. 
Some  products  of  living  activity  not  infrequently  become  a 


20  LIVING  AND  LIFELESS  MATTER 

poison  to  the  organisms  which  secrete  them,  or  to  other  animals 
which  absorb  them  in  one  way  or  another. 

MOVEMENT. — Appearance  and  form  thus  cannot  be  unmis- 
takable symbols  of  living  matter.  No  mistake  can  be  made, 
however,  if  these  two  manifestations  of  vitality  are  taken  to- 
gether with  a  third  and  most  important  one,  viz.  movement. 
A  questionable  object,  if  it  has  the  characteristic  form  and 
appearance  of  protoplasm  and  combines  with  these  the  power 
of  independent  movement,  may  be  safely  interpreted  as  living 
matter.  Movement  alone  is  not  sufficient,  for  lifeless  matter 
may  exhibit  spontaneous  movements  of  one  kind  or  another.  A 
drop  of  water,  for  example,  on  a  hot  surface  will  move  with 
characteristic  activity,  or  a  piece  of  camphor  on  the  surface  of 
clean  water  will  dance  about  with  considerable  vigor,  while 
emulsions  of  oil,  water,  and  salt  will  not  only  simulate  the  ap- 
pearance of  protoplasm  but  will  also  imitate  the  movements  of 
certain  kinds  of  lower  animals.  In  all  of  these  cases,  however, 
movement  is  not  spontaneous  and  independent,  originating 
from  within  the  substance,  but  is  due  to  surface  tension  or  the 
interaction  of  the  more  fluid  water  and  the  less  fluid  substance, 
and  is  explained  on  purely  physical  grounds.  Movement  of 
living  things,  while  it  may  have  at  bottom  some  similar  physi- 
cal principle,  is  quite  different  for  it  originates  through  the 
liberation  of  energy  within  the  living  substance. 

The  types  of  movement  of  living  things  are  quite  varied  but 
they  may  all  be  referred  to  one  or  the  other  of  the  following 
kinds:  (i)  flowing  movement;  (2)  amoeboid  movement;  (3) 
ciliary  movement  and  (4)  muscular  contraction. 

Flowing  Movement. — The  cells  of  the  stone  wort  (Nitella)  are 
elongate  units  of  structure  with  heavy  walls  of  cellulose.  With- 
in the  walls  a  steady  streaming  of  granules  can  be  made  out. 
This  flow  is  confined  to  the  layer  of  protoplasm  around  the 
periphery  of  the  cell  just  within  the  cellulose  membrane,  the  cen- 
ter of  the  cell  being  filled  by  a  large  vacuole  containing  water 
which  presses  the  living  substance  (primordial  utricle)  against 
the  walls.  The  protoplasmic  flow  is  rendered  visible  by  the 
presence  of  larger  or  smaller  granules  and  of  "nuclei, "  which  are 


PROTOPLASMIC  MOVEMENTS 


21 


continually   swept  up   and   down   in   the   ever  moving  mass 
(Fig.  9). 


FIG.  9. — A,  Two  cells  and  a  part  of  a  third  from  a  stonewort  (Nitella)  showing 
rotation  in  the  direction  of  the  arrows,  m,  Membrane  of  the  cell;  n,  nucleus. 
B  and  C,  cells  from  the  stamen  hairs  of  the  spiderwort  (Tradescantia)  showing 
circulation  of  protoplasm  as  indicated  by  the  arrows.  (From  Sedgwick  and 
Wilson.) 

Another  type  of  flowing  movement  may  be  seen  in  the  stamen 
hairs  of  the  spiderwort  (Tradescantia)  which  consist  of  single 
rows  of  cells.  Not  only  is  there  a  flowing  of  granules  and  proto- 


22  LIVING  AND  LIFELESS  MATTER 

plasm  around  the  walls,  but  streams  of  flowing  protoplasm 
reach  into  and  pass  through  the  central  cavity  so  that  a  more  or 
less  perfect  circulation  occurs  (Fig.  9  B). 

Amoeboid  Movement. — In  flowing  movement  the  fluid  proto- 
plasm moves  more  or  less  briskly  according  to  the  temperature, 
but  it  is  usually  kept  within  bounds  by  the  firm  lifeless  cell  walls. 
A  free  living  organism  without  such  walls  might  be  expected  to 
move  in  any  direction  and  without  restraint.  Such  a  form  is 
Amoeba  proteus,  a  small  animal  found  in  stagnant  pools  and 
consisting  of  one  cell  only.  Here  the  protoplasmic  granules  are 
almost  always  in  motion,  and,  having  no  firm  covering,  the 


FIG.  10. — Different  forms   assumed  by  Amoeba   proteus.     Photographs   from 

preparations. 

periphery  gives  way  and  a  line  of  flow  is  started  in  the  direction 
of  the  outbreak.  This  flow  continues  until  the  forces  which 
caused  the  rupture  are  expended,  or  until  some  point  offering 
less  resistance  gives  way  and  a  new  line  of  flow  is  started.  In 
this  way  the  bulk  of  the  minute  organism  moves  about  in  the 
water,  its  form  constantly  changing  the  while  (Fig.  10). 

This  amoeboid  motion  is  not  uncommon  in  certain  cells  of 
higher  animals,  especially  in  the  white  blood  cells  or  leucocytes. 

Ciliary  Movement. — In  both  flowing  and  amoeboid  movement 
the  source  of  energy  probably  lies  in  the  chemical  processes 


PROTOPLASMIC  MOVEMENTS  23 

which  are  transpiring  all  of  the  time  and  in  all  parts  of  the 
protoplasmic  substance.  In  another  type  of  movement,  termed 
ciliary  movement,  the  main  liberation  of  energy  is  apparently 
confined  to  one  region  of  the  cell  or  to  specialized  parts  of  the 
protoplasm  of  that  cell.  Manifestations  of  the  liberated  energy 
are  expressed  solely  by  such  specialized  portions  or  by  out- 
growths from  them.  These  outgrowths,  known  as  fiagella  and 
cilia,  are  minute  whip-like  processes  of  the  cell  which  undulate 
in  the  surrounding  medium  or  lash  it  like  an  oar.  Fiagella  are 
usually  single  or  at  most,  few  in  number,  but  cilia  are  numerous 
and  their  beating  moves  the  cells  with  considerable  rapidity 
if  they  are  free,  or  creates  currents  in  the  surrounding  medium  if 
the  cells  are  fixed.  Cilia  thus  play  an  important  part,  some- 
times as  in  protozoa  and  larval  forms  of  invertebrates,  in  loco- 
motion, sometimes  as  in  the  ciliated  cells  of  various  ducts,  in 
creating  currents  in  the  surrounding  media.  Thus  the  ciliated 
cells  of  the  trachea  sweep  particles  of  dust,  mucus,  etc.,  to  the 
outside.  For  this  purpose  the  stroke  of  the  cilia  is  upward,  and 
is  much  stronger  than  the  recovery.  Fiagella  have  an  entirely 
different  type  of  motion,  acting  with  a  cork-screw  or  sculling 
movement.  These  are  rarely  found  in  higher  animals  save  as 
the  motile  organs  of  spermatozoa,  but  are  characteristic  of  many 
unicellular  animals  and  of  many  plant  zoospores  and  motile 
gametes. 

Muscular  Contraction. — The  most  highly  specialized  type  of 
movement  of  living  protoplasm  is  undoubtedly  muscular 
contraction.  This  is  limited  to  special  cells  of  fibrous  nature 
which  are  usually  bound  together  in  bundles,  thus  forming 
muscles.  Upon  irritation  a  stimulus  is  transmitted  by  a  nerve 
to  the  muscle  cells,  and  contraction  results.  In  this  contraction 
the  bulk  of  the  muscle  cell  remains  the  same  but  the  form 
changes,  the  muscle  bundle  becoming  shorter  and  thicker  (Fig. 
n).  Muscular  action,  in  higher  animals  at  least,  is  usually  the 
sole  means  of  locomotion  from  place  to  place;  in  these  animals 
the  muscles  usually  connect  movable  joints  with  fixed  parts  of 
the  skeleton.  The  majority  of  muscles  are  under  the  control 
of  the  organism  and  can  be  moved  at  will.  These,  the  volun- 


24 


LIVING  AND  LIFELESS  MATTER 


tary  muscles,  are  used  mainly  for  locomotion,  food  getting  and 
other  ordinary  purposes  in  the  life  of  the  individual.  Others, 
the  involuntary  muscles,  like  those  of  digestive  tract  and  heart, 
work  automatically. 

All  three  of  these  manifestations  of  vitality  are  closely  con- 
nected. Form  and  appearance  of  protoplasm  are  largely  de- 
pendent upon  movement  of  the  protoplasmic  mass,  whole  or  in 
part.  Movement  of  any  type,  in  turn  is  dependent  upon  the 
conditions  of  the  surrounding  medium,  or  the  environment. 
It  is  a  general  truth  that  heat  accelerates  and  cold  diminishes, 
all  within  certain  limits,  the  activities  of  protoplasm.  The 
protoplasm  of  an  Amoeba  or  of  Nitella,  the  cilia  of  an  epithe- 
lium, move  faster  with  a  slight  increase  in  temperature.  Reduc- 


i 

i,'iirnriin 

l'liT/.<;t 


fig 
. 

. 


FIG.  ii.— Finer  structure  of  a  muscle  cell  (on  left)  and  change  of  form  of  a 
muscle.  A,  at  rest;  B,  contracted,  p,  Protoplasm;  w,  nucleus.  (From  Sedgwick 
and  Wilson.) 

tion  of  temperature,  on  the  other  hand,  retards  these  move- 
ments, until  with  increasing  cold  there  comes  a  temperature  in 
which  all  activity  ceases.  Most  organisms  are  destroyed  at  the 
temperature  of  boiling  water,  although  by  special  adaptations 
some  are  able  to  withstand  a  much  higher  temperature  (bacteria 
spores).  High  temperatures  cause  the  coagulation  of  certain 
substances  in  protoplasm,  and  lead  to  what  is  called  heat  rigor 
(rigor  caloris),  usually  between  40°  and  50°  C.  There  is  no 


PROTOPLASMIC  MOVEMENTS  25 

general  rule  in  regard  to  the  most  favorable  or  optimum  tem- 
perature for  all  living  things;  some  animals  and  plants,  which 
are  adapted  to  life  in  arctic  circles  or  in  the  depths  of  the 
ocean,  would-  die  in  warmer  climates,  and  vice  versa. 


CHAPTER  II 

PROTOPLASM  AND  THE  CELL,  AND  ORGANISMS  OF 

ONE  CELL 

IF  living  things  consisted  solely  of  protoplasm,  the  larger 
forms  at  least  would  appear  little  more  than  amorphous  masses 
of  jelly.  We  have  seen,  however,  that  lifeless  matter  accompa- 
nies living  substance,  and  is  laid  down  by  this  substance  to  form 
supporting  structures  of  one  kind  or  other.  Such  larger  forms 
are  made  possible,  furthermore,  by  the  fact  that  the  protoplasm 
is  divided  up  into  innumerable  units  of  structure  termed  cells, 
each  one  of  which  has  its  own  cell  wall  which  is  more  or  less 
firmly  attached  to  adjacent  cells,  thus  giving  mutual  support 
and  solidarity  to  the  whole.  Lifeless  matter  deposited  in 
and  around  these  cells  adds  further  support  and  strength 
(Fig.  12). 

Robert  Hooke,  an  English  botanist,  in  1665,  after  studying 
the  structure  of  wood  and  higher  plants,  came  to  the  conclusion 
that  cork  and  wood  generally  is  made  up  of  minute  boxes  which 
he  termed  Cells.  He  believed  that  the  walls  were  the  essential 
parts  of  the  cell  since  the  contents  seemed  invariably  absent. 
As  microscopes  improved,  this  conception  of  the  finer  structure 
of  living  things  became  widely  recognized,  until  in  1838-40  the 
botanist  Schleiden  and  the  zoologist  Schwann  announced  their 
belief  that  all  plants  and  all  animals  are  composed  of  minute 
units  of  structure  to  which,  following  Hooke,  they  gave  the 
name  cells.  But  even  they  did  not  have  the  idea  of  cells  that  we 
have  today  but  regarded  the  walls  as  the  vital  parts.  Proto- 
plasm, as  the  fundamental  living  substance,  was  practically 
unknown,  although  in  1835  a  French  naturalist  Felix  Dujardin 
studied  the  structure  of  certain  foraminifera  or  naked  bits 
of  living  matter  without  cell  walls  and  published  his  conclusion 

26 


CELL  STRUCTURE 


27 


that  this  living  substance,  which  he  called  "sarcode,"  is  a 
simpler  form  of  living  matter  than  that  which  composes  the 
bodies  of  higher  animals  or  plants.  It  was  not  until  1863  that 
protoplasm  and  sarcode  were  shown,  by  Max  Schultze,  to  be 
the  same  type  of  substance.  In  the  meantime  research  on  the 
finer  structure  of  different  animals  and  plants  extended  the 
cell  theory  of  Schleiden  and  Schwann  to  form  after  form,  while 
the  older  view  that  the  walls  are  the  essential  parts  was  gradu- 


. '  K-e-f  *-jw^»<,  '  fr '  'f "  I 

FIG.  12. — General  view  of  cells  composing  the  growing  root- tip  of  an  onion; 
some  cells  in  stages  of  division  (mitosis,  see  p.  209).  a,  Non-dividing  cells;  b, 
early  stages  of  nuclear  change;  c,  cells  in  full  mitosis.  (From  E.  B.  Wilson.) 

ally  replaced  by  the  modern  conception  of  protoplasm  and  cell 
structure. 

It  thus  follows  that  the  term  cell,  meaning  originally  an 
empty  box,  then  a  framework  with  fluid  contents,  has  come  to 
mean  finally  a  small  unit  mass  of  living  material,  while  the  cellu- 
lar structure  of  all  living  things  has  no  longer  the  uncertain 
standing  of  a  theory,  but  is  one  of  the  fundamental  and  firmly 
established  facts 'of  biology. 


28  PROTOPLASM  AND  THE  CELL 

All  cells  have  the  same  general  structure;  all  are  composed  of 
protoplasm,  of  which  a  part,  called  the  nucleus,  is  different  from 
the  remainder  of  the  cell  body,  or  cytoplasm.  In  the  higher 
types  of  living  things,  where  innumerable  cells  make  up  the 
body  of  the  individual,  the  cells  are  specialized  to  perform 
different  functions.  Groups  or  sheets  of  similar  cells,  perform- 
ing a  like  function  or  functions,  are  called  tissues,  and  aggregates 
of  different  tissues  for  the  performance  of  some  one  function 
are  called  organs,  whence  the  term  organism.  In  both  animal 
and  plant  kingdoms  there  are  individuals  consisting  of  one  single 
cell;  these  are  known  as  the  unicellular  organisms  and  may 
be  unicellular  animals  or  unicellular  plants;  if  animals  they  are 
called  Protozoa,  if  plants,  Protophyta.  Higher  in  the  scale  we 
find  animals  on  the  one  hand  and  plants  on  the  other,  consisting 
of  tissues  only — the  sponges  and  coelenterates  among  animals, 
and  some  types  of  Thallophytes  among  plants.  Still  higher 
in  the  scale,  finally,  are  organisms  consisting  of  organs,  the 
highest  types  of  living  things.  In  the  following  pages  we  will 
consider  first  the  organisms  of  one  cell,  then  organisms  of 
tissues,  and  finally  organisms  of  organs. 

The  fundamental  vital  functions  are  performed  by  all  living 
things  but  there  is  a  great  difference  in  the  complexity  of  organs 
for  the  performance  of  such  vital  activities.  We  speak  of  organ- 
isms as  generalized  when  all  of  the  physiological  activities  are 
performed  by  a  relatively  few  organs,  and  as  specialized  when 
each  of  the  necessary  activities  is  distributed  among  a  number  of 
organs,  each  organ  contributing  a  part.  With  man  and  the 
mammals,  specialization  has  gone  the  farthest;  special  organs 
composed  of  many  tissues,  each  tissue  of  a  congeries  of  similar 
cells,  perform  the  vital  activities.  Each  organ  contributes  its 
activity  or  product  to  the  aggregate  or  individual,  and  all  or- 
gans act  in  harmony  for  the  good  of  the  whole.  This  phenome- 
non of  dividing  the  necessary  activities  among  many  parts  is 
analogous  to  division  of  labor  in  human  communities,  and  is 
called  the  division  of  physiological  labor.  With  animals  at 
the  other  extreme  of  the  animal  scale  from  man,  all  of  the  vital 
processes  are  performed  by  the  protoplasm  comprising  only  one 


YEAST  29 

cell.  Here  organic  structures  are  reduced  to  their  lowest  terms, 
but  the  functional  activities  are  the  same  in  essence  as  in  higher 
animals,  and  the  unicellular  protozoon  is  a  complete  organism, 
vitally  no  less  complete  than  a  bird,  fish  or  mammal.  The 
several  grades  in  complexity  present  different  aspects  of  bio- 
logical principles  and  justify  our  division  into  organisms  of  one 
cell,  organisms  of  tissues,  and  organisms  of  organs. 

The  physiological  activities  of  single-celled  organisms,  while 
undoubtedly  simpler  than  those  of  many-celled  animals,  are 
nevertheless  so  marvelously  complicated  that  only  a  little  ad- 
vance in  knowledge  has  been  made.  To  this  advance  the 
minute  organisms  known  as  the  yeasts  have  contributed  no 
small  part. 

A.  THE  ORGANIZATION  AND  VITALITY  OF  YEAST  CELLS 

Yeasts  are  widely  spread  in  nature,  occurring  either  as  "wild" 
forms  or  as  cultivated  commercial  types.  Wild  yeasts  live 
normally  upon  the  surfaces  of  fruits  of  various  kinds,  or  on 
fruit  juices;  in  addition  to  this  habitat,  however,  there  are  a 
great  many  wild  yeasts  that  live  normally  in  the  digestive  tract 
or  body  cavities  of  different  kinds  of  animals.  A  drop  of  sweet 
cider  gives  a  good  idea  of  certain  species.  Baker's  yeast,  mixed 
with  water,  shows  myriads  of  minute  yeast  cells  which  cause 
the  milky  appearance  of  the  fluid.  Brewer's  yeast  contains 
several  species,  two  well-marked  kinds  form  the  "top"  and 
"bottom"  yeast  of  commerce,  the  former  being  used  for  the 
manufacture  of  ales,  stout,  porter,  etc.,  the  latter  for  "lager 
beer." 

Microscopical  examination  of  baker's  or  brewer's  yeast  shows 
that  the  individual  yeast  cells  have  but  little  structure.  A  tiny 
bit  of  gray  protoplasm  is  enclosed  in  a  definite  double-contoured 
membrane  which,  by  appropriate  treatment,  may  be  shown  to 
consist  of  cellulose.  The  cells  are  spherical  or  spheroidal  in 
form,  and  the  protoplasm  contains,  in  addition  to  the  ordinary 
granules  of  protoplasm,  small  or  larger  refractile  dots  probably 
of  the  nature  of  fat.  Several  vacuoles  are  present  and  a  nucleus. 


30 


PROTOPLASM  AND  THE  CELL 


All  of  these  constituents  of  the  cell  vary  according  to  conditions. 
In  old  cells  the  cellulose  membrane  is  thicker  than  in  young 
cells,  as  can  be  easily  demonstrated  by  the  use  of  aqueous  solu- 
tion of  magenta.  The  nucleus  is  larger  and  more  conspicuous 
in  large  cells  and  may  be  found  in  the  process  of  division  (Fig. 
13).  Fat  droplets  and  vacuoles,  also,  vary  in  number  and  size 
according  to  the  conditions. 


FIG.  13. — Yeast  cells  showing  nuclei  and  successive  stages  in  the  process  of 
budding.     (From  Sedgwick  and  Wilson.) 


REPRODUCTION 

Budding. — In  active,  growing  yeast  it  is  very  easy  to  find 
cells  in  the  process  of  reproduction.  This  is  brought  about  by 
budding  or  gemmation,  which  begins  with  a  local  swelling  usu- 
ally at  one  pole  of  the  spheroidal  cell.  The  membrane  appears 
to  give  way  or  to  weaken  at  one  point,  and  the  inner  protoplasm 
presses  into  this  region,  forcing  out  the  thinned  membrane,  until 
a  well-marked  bud  is  formed.  Later  this  bud  is  constricted  off 
and  it  becomes  a  separate,  young,  yeast  cell.  Frequently  the 
bud  continues  to  grow  until  mature  without  breaking  away  from 


YEAST 


31 


the  parent  cell,  and  may  even  bud  in  turn,  thus  giving  rise  to 
chains  of  yeast  cells  (Fig.  14). 

Spore-formation. — Another  mode  of  reproduction  occurs 
under  certain  and  for  the  most  part  unknown  conditions. 
The  protoplasm  divides,  within  the  cellulose  membrane,  to 
form  two,  three,  or  four  compact,  rounded  spores  (Fig.  15). 
Under  favorable  conditions  the  spore  capsules  burst  or  sprout, 
and  the  spores  emerge  as  yeast  cells  which  then  develop  like 


FIG.   14. — Colonies  of  budding  yeast  cells.     (From  Sedgwick    and    Wilson.) 


ordinary  forms.  Reproduction  by  this  means  is  called  endo- 
genous sporulation,  which  differs  somewhat  from  "spore- 
formation"  in  bacteria  where  there  may  be  no  actual  reproduc- 
tion but  merely  a  temporary  protection  against  drying,  or 
other  unfavorable  condition  of  the  environment. 

Culture  Media. — The  simplicity  of  structure  of  yeast  cells 
would  naturally  suggest  a  simplification  of  the  vital  processes, 
and  lend  support  to  the  belief  that  these  might  be  more  readily 


32 


PROTOPLASM  AND  THE  CELL 


analyzed  than  can  vital  processes  in  higher  types  of  cells. 
This  belief,  indeed,  has  been  realized  to  a  certain  extent, 
although  the  secrets  of  constructive  and  destructive  meta- 
bolism are  still  unrecognized.  The  sweet  fluids  of  fruits 
offer  an  excellent  medium  in  which  yeast  cells  grow  and 
multiply.  An  even  more  excellent  medium  is  prepared  from 
the  proteins,  sugar  and  salts  extracted  from  the  young  cells 
of  sprouting  barley.  This  medium,  known  as  sweet  wort,  sup- 
plies the  necessary  elements  for  the  living  protoplasm  of  yeast, 
and  the  vital  processes  go  on  at  a  rapid  rate.  Sweet  wort, 
however,  and  the  sugary  juices  of  fruits  are  too  complex  to 
give  any  more  adequate  notion  of  the  food  value  of  specific 
elements,  than  would  the  protein  food  of  higher  forms  of  life. 
Fortunately,  however,  the  yeast  processes  are  so  primitive  that 
more  direct  and  exact  knowledge  is  possible. 

If  a  quantity  of  pure  yeast  is  burned,  the  mass  first  chars  by 
the  deposit  of  carbon,  then,  with  continued  heat,  this  is  used 


FIG.  15. — Endogenous  spore  formation  in  yeast.     (From  Sedgwick  and  Wilson.) 

up  in  forming  carbon  dioxide  by  union  with  oxygen,  while  at 
the  same  time  the  nitrogen  of  the  yeast  is  given  off  in  the  form 
of  nitrogen  gas,  hydrogen  as  water  vapor,  and  sulphur  as  sul- 
phurous acid  or  sulphur  dioxide.  Finally  nothing  remains  but 
a  white  ash  composed  of  potassium,  lime,  magnesium,  and 
phosphoric  acid. 

Pasteur  made  up  a  fluid  composed  of  the  ingredients  thus 
obtained  by  analysis,  and  found  that  yeast  cells  would  grow 
and  multiply  in  it  as  in  sweet  wort.  With  such  a  fluid  he  was 


YEAST  METABOLISM  33 

able,  by  omitting  one  substance  after  another,  to  determine 
what  elements  are  necessary  for  the  vital  activities  of  yeast. 
The  fluid,  known  as  Pasteur's  fluid,  has  the  following  percentage 
composition: 

Water  H2O 83 . 76  per  cent. 

Cane  sugar  (Ci2H22Oii) 15.00  per  cent. 

Ammonium  tartrate  (NH^C-iH^Oe i  .00  per  cent. 

Potassium  phosphate  K3PO4 20  per  cent. 

Calcium  phosphate  Ca3(PO4)2 • 02  per  cent. 

Magnesium  sulphate  MgSC>4 02  per  cent. 

Bearing  in  mind  the  essential  elements,  C,  H,  N,  O,  P,  S, 
and  some  salts,  found  in  all  protoplasm  it  will  be  seen  that  the 
ingredients  of  Pasteur's  solution  contain  all  of  the  needed  ele- 
ments. On  a  priori  grounds  it  would  be  possible  to  leave  out 
some  of  the  ingredients  without  seriously  affecting  the  vital 
reactions.  If  sugar,  for  example,  is  omitted,  all  of  the  elements 
which  it  contains  are  found  in  the  other  ingredients  and  the 
yeast  cells  continue  to  grow  and  multiply  although  fermentation 
ceases.  If  some  salts  are  left  out,  growth  is  much  retarded  and 
vital  actions  are  slow.  The  one  absolutely  essential  ingredient 
is  the  ammonium  tartrate.  If  this  is  omitted,  life  processes 
cease  altogether,  and  a  glance  at  the  chemical  symbols  shows 
that  this  alone  contains  nitrogen. 

By  means  of  this  simplified  medium  it  is  demonstrated  that 
yeast  cells  are  much  less  complex  in  their  nutritive  processes 
than  green  plants  on  the  one  hand  and  animals  on  the  other. 
Green,  or  chlorophyll-bearing  plants,  by  photosynthesis  (see 
p.  119),  manufacture  food  from  far  simpler  elements  than 
proteins;  animals  require  proteins  ready  made,  and  these,  as 
we  have  seen,  are  highly  complex  substances.  Yeasts  survive 
and  thrive  on  a  nitrogenous  compound  much  less  complex  than 
protein  and  more  complex  than  CO2  and  H2O  which  serve  as 
food  elements  of  green  plants;  they  are,  therefore,  as  regards 
nutrition  at  least,  intermediate  between  such  plants  and 
animals. 

Even    with    this    simplified    nutrition,   however,   the    finer 


34  PROTOPLASM  AND  THE  CELL 

processes  of  assimilation  and  the  upbuilding  of  yeast  proto- 
plasm, are  as  obscure  as  elsewhere  in  the  living  world,  but, 
largely  through  the  study  of  yeast  cells,  some  good  working 
hypotheses  have  been  formulated  and  many  vital  activities 
have  been  traced  to  unstable  chemical  compounds  termed 
enzymes  or  ferments. 

B.  BACTERIA 

Bacteria  is  a  term  used  to  designate  a  great  group  of  minute 
forms  of  life  intermediate,  like  yeast,  between  chlorophyll- 
bearing  plants  and  animals.  Bacteria  occur  almost  everywhere; 
abundantly  in  the  atmosphere  accompanying  dust  particles; 
frequently  in  fresh  and  salt  water;  abundantly  in  the  digestive 
tract  of  all  kinds  of  animals.  They  abound  in  the  upper  layers 
of  the  soil  and  in  exposed  fluids  containing  dead  animal  or 
vegetable  matter.  Some  types  produce  disease  in  man  and 
other  animals,  whence  they  are  popularly  known  as  germs  or 
microbes  or  parasites.  Many  of  them,  on  the  other  hand,  are 
positively  useful  physiologically,  in  the  functional  activities  of 
higher  animals,  and  economically  and  commercially  in  trans- 
forming organic  matter  into  simple  salts  (nitrites,  nitrates,  etc.) 
or  in  the  manufacture  of  various  food  stuffs  (vinegar,  butter, 
cheese,  etc.). 

Morphology  of  Bacteria. — Bacteria  are  the  smallest  of  the 
known  organisms.  Some  types  placed  end  to  end  would 
require  25,000  to  cover  a  linear  inch,  and  the  line  would  be  too 
fine  to  be  seen;  50,000  such  lines  side  by  side  would  cover  a 
square  inch.  Other  types  are  larger,  varying  from  2ju  to  60 ju 
in  length.1 

While  small,  the  bacteria  nevertheless  have  fairly  definite 
forms  which  may  be  grouped  for  convenience  under  three  main 
types:  i.  the  bacillus  or  rod;  2.  the  coccus  or  ball;  and  3.  the 
spiral  or  corkscrew.  They  are  frequently  united  in  chains  or 
filaments,  in  plate  form  (sarcina),  or  embedded  in  a  gelatinous 
matrix  which  they  secrete  (zoogloea)  (Fig.  16). 

1  A  ju    (micron)  equal  1-25,000  of  an  inch. 


BACTERIA 


35 


The  bacteria  are  usually  regarded  as  single-celled  organisms 
although  the  complete  cell  structure  is  rarely  present.  The 
majority  have  no  cell  nucleus  but  contain  from  one  to  many 
granules  of  chroma  tin  distributed  throughout  the  cell;  these 
granules  correspond  to  the  nuclei  of  tissue  cells  (Fig.  16,  D). 
The  cells  are  enclosed  in  firm  cell  membranes,  probably  com- 
posed of  cellulose  or  an  allied  substance,  which  are  unbroken  ex- 
cept in  a  small  number  of  forms  provided  with  flagella. 


O 


B    0 


m 

FIG.  16. — Comparative  size  of  A,  human  blood  corpuscle,  B,  typhoid  bacillus, 
C,  influenza  bacillus,  D,  giant  bacillus  from  the  intestine  of  a  cockroach,  and  E, 
a  common  water  spirillum. 


Reproduction. — All  bacteria  multiply  by  transverse  division 
of  the  cell  (Fig.  17).  Division  is  followed  by  rapid  growth,  and 
cycles  of  growth  and  division  follow  one  another  in  quick  suc- 
cession (hay  bacillus  30  minutes,  cholera  vibrio  20  minutes). 
"It  has  been  estimated  that  if  bacterial  multiplication  went  on 
unchecked,  and  the  division  of  each  cell  took  place  as  often  as 
once  an  hour,  the  descendants  of  each  individual  would  in  two 
days  number  281,500,000,000,  and  that  in  three  days  the  prog- 
eny of  a  single  cell  would  balance  148,356  hundredweight!" 
(Jordan,  General  Bacteriology,  p.  61.)  Such  increase  does  not 
take  place  in  nature,  however,  because  of  various  external  in- 


36 


PROTOPLASM  AND  THE  CELL 


fluences  as  well  as  internal  influences  produced  by  the  bacteria 
themselves.  The  environment  is  soon  changed  because  of  their 
own  physiological  activities  and  multiplication  is  soon  checked. 
Reproduction  is  quickly  stopped  by  natural  factors  like  dessi- 

cation,  unsuitable  temperature,  acid- 
ity or  alkalinity  of  the  medium,  but 
many  bacteria   have   the  power   to 
resist    such    adverse    conditions    by 
forming  internal  spores  or  Dauers- 
G     poren  (enduring  spores).     These  are 
FIG.    17—  Spore   formation   usually  spherical,  ellipsoidal  or  oval 

and   germination  of  spores   in    ...  .  _  . 

bacteria.    A,  A  pair  of  rods  in  form  and  possess  a  dense  envelope 


hour  later;  C,  one  hour  later  of  the  chromatin  granules  and  some 

still;  D,  a  five-celled  rod  with  •,  /^.  -.-^^      m, 

three   ripe  spores   which   were    Cytoplasm  (Fig.  17,  D).    These  spores 

placed  in  a  nutrient  medium  possess  a  much  higher  resistance  to 

after  drying  for  several  days;    r 

E,  F,  the  same  spores  from  one  external  influences  than  do  the  cells 
o?  ^£S&3$*  "and  ^om  which  they  are  formed  (many 
movement.  (From  de  Bary  for  example,  can  withstand  a  tern- 
after  Sedgwick  and  Wilson.)  0  N 

perature    of    from    70  to  100    C.). 

One  spore  per  cell  is  the  rule,  but  in  rare  instances,  two  similar 
spores  may  be  formed.  Spore  formation  in  bacteria,  therefore, 
is  not  always  a  method  of  reproduction  but  may  be  an  adaptation 
for  the  preservation  of  the  organism  corresponding  to  what 
is  known  as  the  "encysted  state"  of  many  unicellular  animals. 
Physiology  of  Bacteria.  —  The  food  of  bacteria  is  most  diverse. 
The  majority  are  known  as  saprophytes,  that  is,  they  obtain 
their  nourishment  from  dead  organic  matter.  Many  are  para- 
sites, getting  their  food  from  other  living  organisms  in  the  form 
of  complex  chemical  compounds  of  protein  substance  or  protein 
derivatives.  Some  live  in  the  soil,  and  get  their  food  supply  and 
their  energy  from  purely  inorganic  materials.  Among  these  are 
the  so-called  nitrifying  bacteria,  one  of  which,  Nitrosomonas, 
converts  ammonia  salts  into  nitrites  while  another,  Nitrobacter, 
changes  the  nitrites  into  nitrates.  Other  bacteria  utilize  free 
ammonia  (NH3)  and  still  others,  free  nitrogen  (N)  in  the  manu- 
facture of  nitrates  (see  p.  129).  These  organisms  thus  per- 


FERMENTATION  37 

form  a  most  useful  economic  function  in  preparing  food  mate- 
rial in  the  soil  for  use  by  the  green  plants.  But  the  chief 
biological  interest  of  these  forms  is  that  they  are  able  to  build  up 
their  own  protein  molecules  directly  from  relatively  simple  sub- 
stances without  the  aid  of  chlorophyll,  and  to  get  energy  from 
such  compounds  in  which  it  is  locked  up  for  all  other  kinds  of 
living  things.  Thus  urea,  thrown  off  by  animals  and  plants  as  a 
useless  and  to  them  harmful  waste  matter,  is  a  source  of  food 
and  energy  for  some  bacteria  which  convert  it  into  free  am- 
monia, carbon  dioxide  and  water. 

C.  ENZYMES,  HORMONES  AND  VITAMINES 

f 

Alcoholic  Fermentation. — The  control  of  alcohol  production 
in  practical  ways  was  well  understood  long  before  the  explana- 
tion of  its  production  was  worked  out.  The  term  fermentation 
was  early  given  to  all  processes  involving  the  generation  of  gas, 
probably  because  of  the  froth  or  foam  which  appears  during 
alcohol  formation  or  when  acids  are  allowed  to  act  on  carbon- 
ates. In  the  i  yth  century,  however,  a  distinction  was  drawn 
between  alcoholic  fermentation  and  acid  fermentation,  and  it 
was  recognized  that  alcohol  is  a  new  product  of  the  fermenta- 
tion process  and  quite  distinct  from  the  gas  mnorum  (CO2) 
arising  at  the  same  time.  In  the  iyth  century  also,  Leeuwen- 
hoek,  the  first  microscopist,  discovered  that  the  scum  or  deposit 
which  is  always  present  during  fermentation  is  made  up  of 
small  spherical  bodies  which  he  did  not  attempt  to  identify  as 
animal  or  plant.  With  Lavoisier  in  the  i8th  century,  chemistry 
became  a  more  exact  science  and,  in  connection  with  alcoholic 
fermentation,  it  was  found  that  the  sugar  in  fermenting  fluids 
breaks  down  into  alcohol  and  CC>2  gas,  with  traces  of  glycerine 
and  acetic  acid.  As  with  most  other  chemical  processes,  fer- 
mentation was  regarded  by  Lavoisier  as  a  process  of  oxidation, 
and  the  spherical  bodies  discovered  by  Leeuwenhoek  were  con- 
sidered unimportant.  Early  in  the  igth  century,  however,  a 
reaction  set  in  against  many  of  Lavoisier's  views,  and  its  effects 
are  seen  in  the  interpretation  of  alcoholic  fermentation.  On 


38  PROTOPLASM  AND  THE  CELL 

the  negative  side,  Schwann  showed  that  oxygen  had  nothing 
to  do  with  the  process.  By  simply  heating  the  air  about  fer- 
mentable fluids  which  had  been  properly  prepared  (sterilized), 
he  found  that  no  fermentation  took  place,  whereas  fermenta- 
tion does  occur  in  such  fluids  if  they  are  exposed  to  the  ordinary 
atmospheric  air.  Schwann  concluded  that  something  in  ordi- 
nary air  is  destroyed  by  heating,  and  fermentation  is  prevented. 
On  the  positive  side  Erxleben  in  1818  suggested  that  the  glob- 
ules discovered  by  Leeuwenhoek  might  be  the  cause  of  fermen- 
tation. This  view  was  elaborated  and  confirmed  by  Cagniard 
de  Latour  in  1835,  and  was  finally  conclusively  proved  by  the 
epoch-making  experiments  of  Pasteur  (1857—1863). 

Yeast  was  thus  proved  to  be  the  agent  of  alcoholic  fermenta- 
tion by  acting  in  some  way  on  the  sugar  contents  of  nutrient 
media.  By  this  action  about  95  per  cent,  of  the  sugar  is  broken 
down  into  alcohol  and  CO2;  about  4  per  cent,  is  decomposed 
with  the  formation  of  glycerine,  succinic  acid,  and  C02;  and 
about  i  per  cent,  is  used  by  the  yeast  cells  as  food.  The  small 
amount  of  acetic  acid  that  is  usually  present  is  not  due  to  the 
activity  of  the  yeast  cells  but  to  oxidation,  through  the  agency 
of  bacteria,  of  the  alcohol  already  formed. 

The  approximate  chemical  reactions  involved  in  the  forma- 
tion of  alcohol  and  acetic  acid  are  as  follows: 

C6H12O6  (sugar)  +  yeast  =  2C2H6O  (alcohol)  +  2CO2 
C2HCO  (alcohol)  +  O2  +  bacteria  =  C2H4O2  (acetic  acid)  +  H2O. 

The  acetic  acid  is  finally  oxidized  to  water  and  CO2  through 
the  action  of  bacteria  again.  Thus 

C2H4O2  +  2O2  +  bacteria  =  2CO2  +  2H2O 

The  next  step  in  the  investigation  of  alcoholic  fermentation 
was  to  determine  just  what  yeast  does  in  effecting  the  transfor- 
mation of  sugar  into  alcohol.  Biichner  in  1897  was  the  first 
to  demonstrate  that  yeast  cells  contain  a  substance  which 
causes  the  same  destruction  of  sugar  that  living  yeast  cells  do. 
This  was  accomplished  by  grinding  yeast  cells  in  diatomaceous 
earth,  then  compressing  the  mass  and  obtaining  a  clear  fluid 


ENZYME  ACTION  39 

free  from  yeast.  The  clear  fluid,  which  he  called  zymase,  ex- 
tracted in  this  manner  from  the  living  yeast,  was  thus  shown  to 
be  the  active  agent  in  alcoholic  fermentation.  Such  agents  in 
chemical  activities  are  called  ferments  or  enzymes  (from  Greek 
en,  in,  and  zyme,  yeast).  Still  later  investigations  have  shown 
that  a  second  enzyme,  called  co-enzyme  and  also  produced  by 
the  yeast  cell,  is  necessary  to  activate  the  zymase. 

Enzymes  in  Vital  Activities. — The  zymase  and  its  co-enzyme, 
which  can  thus  be  extracted  from  the  protoplasm  of  yeast, 
are  normal  products  of  the  vital  or  metabolic  activities  of  the 
organism  and  are  examples  of  the  many  analogous  ferments 
which  yeast  is  capable  of  producing.  Macfadyen,  Morris,  and 
Rowland  as  well  as  other  investigators,  have  devised  methods 
of  cutting  up  minute  organisms  in  mass,  thus  breaking  down 
the  cells  more  perfectly  than  had  been  done  before.  In  this 
way  it  has  been  possible  to  isolate  from  yeast  not  only  a  powerful 
zymase  but  other  enzymes  as  well,  including  maltase,  invertase, 
endotryptase,  rennin,  and  traces  of  two  others,  all  of  which 
must  be  present  in  the  protoplasm  of  normal  yeast  cells. 

One  of  the  characteristics  of  the  chemical  activities  in  proto- 
plasm which  distinguishes  them  from  similar  activities  in 
physical  nature  is  the  speed  with  which  they  take  place.  Thus 
sugar  dissolved  in  water  and  exposed  to  the  air  oxidizes  very 
slowly.  There  is  no  difference  in  kind  between  this  process 
and  the  oxidation  of  sugar  in  the  living  cell,  but  there  is  a  great 
difference  in  speed.  Such  differences  in  the  rapidity  of  chem- 
ical actions  in  living  and  lifeless  matter  are  due,  as  we  now 
know,  to  the  presence  of  innumerable  enzymes  in  all  kinds  of 
living  cells.  The  first  hint  of  these  elusive  agents  in  vital 
activities  was  given  as  early  as  1836  by  Berzelius  when  he  dis- 
covered the  action  due  to  what  he  called  "catalytic  force." 
Subsequent  researches  have  shown  that  his  catalyzers,  which 
we  now  know  as  enzymes,  are  chemical  substances  which  par- 
ticipate in  chemical  reactions  by  forming  compounds  of  unstable 
and  intermediate  character.  The  compounds  break  down 
easily,  thus  freeing  the  enzymes  and  enabling  them  to  repeat  the 
process,  so  that  they  appear  to  be  unaltered  by  the  reactions 


40  PROTOPLASM  AND  THE  CELL 

which  they  undergo.  These  enzymes  are  probably  organic 
bodies  or  compounds  of  which  the  exact  composition  is  unknown, 
although  for  certain  starch-dissolving  enzymes  (amylases), 
Mathews  concludes  that  "the  indications  are  that  the  active 
part  of  the  molecule  is  a  protein,  probably  colloidal,  and  that 
this  active  principle  is  usually  combined  with  a  colloidal,  car- 
bohydrate gum"  (Physiological  Chemistry,  p.  330). 

While  the  chemical  composition  of  enzymes  is  at  present 
unknown,  we  have  a  rapidly  accumulating  fund  of  information 
concerning  their  classification  and  activities  in  animal  organ- 
isms. Thus  in  the  general  function  of  nutrition  where  the 
digestive  enzymes  are  hydrolytic  agents  throughout,  we  find 
some  that  are  proteoly tic  (i.e.,  that  break  down  proteins) ,  some 
lipolytic  (i.e.,  that  destroy  fats),  and  some  that  are  amylolytic 
(i.e.,  that  break  down  carbohydrates). 

Some  of  the  more  important  proteolytic  enzymes  are:  (i)  pepsin,  which 
acts  in  an  acid  medium  to  break  down  proteins  into  peptones;  (2)  trypsin 
which  acts  in  an  alkaline  medium  to  break  down  proteins  and  to  transform 
products  of  peptic  digestion  into  amino-acids  and  polypeptids;  and  (3) 
erepsin  which  splits  peptones  and  polypeptids  to  amino-acids.  Among  the 
fat-transforming  enzymes,  i.e.,  enzymes  which  split  fats  to  form  glycerine 
and  fatty  acids,  are  different  forms  of  lipase  from  stomach,  intestines  and 
the  pancreas  (steapsin).  Among  the  more  important  amylolytic  ferments 
are  the  amylases  or  diastases  which  convert  insoluble  starch  into  soluble 
sugars,  such  as  ptyalin  of  the  saliva,  diastase  of  the  pancreatic  juice,  and 
other  amylases  of  the  digestive  fluids.  Here  also  belong  the  enzymes 
which  transform  disaccharid  sugars  into  monosaccharids,  such  as  maltase 
of  the  saliva,  which  splits  maltose;  invertase  of  the  stomach  and  pan- 
creatic juices,  which  splits  cane  sugar;  and  lactase  of  the  stomach  and 
pancreatic  juices,  which  splits  milk  sugar. 

Enzymes  having  to  do  with  the  digestion  of  food  substances 
may  act  either  within  the  cells  of  the  body  (phagocytes,  proto- 
zoa and  coelenterates) ,  or  in  cavities  lined  by  the  cells  which 
secrete  them.  In  addition  to  these  digestive  enzymes  there  are 
many  others  in  protoplasm  which  have  to  do  with  the  various 
processes  of  constructive  and  destructive  metabolism,  and  these 
always  act  within  the  cell,  hence  they  are  often  called  the 


HORMONES  41 

endoenzymes.  They  are  best  known  in  connection  with  de- 
structive metabolism,  the  modern  conception  being  that  endo- 
enzymes are  the  causes  of  a  series  of  progressive  chemical  decom- 
positions. Each  chemical  process  is  presided  over  by  a  specific 
endoenzyme  which  acts  only  on  certain  chemical  substances  and 
gives  rise  to  other  chemical  substances  to  be  acted  on  in  turn  by 
other  enzymes.  "The  processes  which  years  ago  were  consid- 
ered as  due  to  the  peculiar  vital  properties  of  the  tissue  cells, 
and  which  were  supposed  to  be  entirely  dependent  upon  their 
morphological  and  functional  integrity,  are  now  seen  to  be  due 
primarily  to  a  great  variety  of  enzymes,  manufactured  indeed 
by  the  living  cells,  but  capable  of  manifesting  their  activity 
even  when  free  from  the  influence  of  the  living  protoplasm. 
The  varied  processes  of  tissue  katabolism  are  the  result  of  or- 
derly and  progressive  chemical  changes,  in  which  cleavage, 
hydrolysis,  reduction,  oxidation,  deamidization,  etc.,  alternate 
with  each  other  under  the  influence  of  specific  enzymes,  where 
chemical  constitution  and  the  structural  make-up  of  the  various 
molecules  are  determining  factors  in  the  changes  produced." 
(Chittenden,  The  Nutrition  of  Mant  pp.  75,  76.) 

Hormones. — A  second  group  of  enigmatical  chemical  sub- 
stances produced  by  living  organisms  includes  the  hormones. 
These  are  extremely  difficult  to  study,  and  facts  regarding  them 
have  come  to  light  only  recently.  A  good  example  is  the  hor- 
mone secretin  in  man,  which  causes  the  pancreas  cells  to  secrete 
the  digestive  ferments  of  the  pancreatic  juice.  This  secretin 
is  formed  after  contact  of  the  acidified  contents  of  the  stomach 
with  the  mucous  membrane  of  the  small  intestine.  The  acid 
food  stuffs  do  not  stimulate  nerves  which  start  secretion  in  the 
pancreas,  but  they  act  apparently  upon  a  substance  formed  in 
the  cells  of  the  mucous  membrane,  transforming  this  substance 
into  a  heat-resisting  hormone  secretin  which  reaches  and  stimu- 
lates the  appropriate  nerves  through  the  blood  and  these,  in 
turn,  stimulate  the  pancreas  cells  to  secrete.  Other  hormones 
are  responsible  for  many  of  the  phenomena  of  growth  and 
differentiation,  and  possibly  they  play  an  important  part  in 
early  development  of  the  individual.  Again,  there  is  strong 


42  PROTOPLASM  AND  THE  CELL  . 

reason  to  suppose  that  secondary  sexual  characteristics  are 
developed  at  maturity  through  the  action"  of  hormones  secreted 
by  the  reproductive  glands  into  the  blood.  A  close  relation 
exists  also  between  the  glandular  patches  known  as  the  corpora 
lutea  on  the  mammalian  ovary,  fixation  of  the  fertilized  eggs  to 
the  wall  of  the  uterus,  and  stimulation  of  the  mammary  glands. 
If  these  small  glands  on  the  ovary  are  removed,  the  developing 
egg  will  not  attach  at  all,  and  it  is  supposed  that  a  hormone  is 
secreted  which  reaches  the  uterus  through  the  blood  and  causes 
the  cells  of  the  uterus  to  react  to  the  embryo.  Again  the 
pituitary  body  in  the  brain  plays  an  important  part  in  regulat- 
ing growth  of  the  organism,  diseases  of  this  gland  giving  rise  to 
acromegaly,  one  of  the  symptoms  of  which  is  the  excessive  de- 
velopment of  parts  of  the  organism  far  removed  from  the  brain. 
Vitamines. — Still  another  group  of  elusive  chemical  bodies  are 
the  so-called  vitamines,  named  by  Casimir  Funk.  Like  enzymes 
and  hormones,  these  are  substances  of  unknown  chemical  com- 
position which  appear  to  be  necessary  for  the  proper  nourish- 
ment of  the  body.  They  are  undoubtedly  organic  in  nature  but 
are  neither  proteins,  carbohydrates,  nor  fats.  They  may  be 
extracted  from  these  normal  foods  by  alcohol,  and  are  destroyed 
by  heat  and  by  alkalies.  They  contain  nitrogen,  but  no  phos- 
phorus, and  are  probably  reducing  substances.  Vitamines  are 
best  known  in  connection  with  the  disease  known  as  Beri-beri 
which  is  caused  by  eating  white  polished  rice -as  a  sole  or  staple 
article  of  diet.  While  such  rice  contains  abundant  nourishment, 
the  body  cannot  utilize  this  nourishment  without  the  aid  of 
vitamines  which  should  go  with  the  rice.  The  outer  coatings  of 
the  rice  kernels,  which  are  removed  in  the  preparation  of  white 
or  polished  rice,  contain  such  vitamines,  and  if  these  husks  are 
eaten  with  the  rice,  a  proper  nourishment  results.  Scurvy 
is  another  disease  which  apparently  results  from  the  absence 
of  vitamines.  All  normal  foods,  whether  proteins,  carbohy- 
drates, or  fats,  contain  these  essential  substances  but  in  dif- 
ferent degrees,  and  all  such  food  substances  can  be  rendered 
innutritious  by  previously  extracting  the  vitamines  while,  on 
the  other  hand,  excellent  results  in  growth  and  increase  in  weight 


VITAMINES  43 

may  be  obtained  by  adding  vitamines  thus  extracted  from 
nutritious  proteins  to  the  ordinary  milk  or  cereal  of  infants 
suffering  from  malnutrition  and  cessation  of  growth  (Eddy). 

From  this  brief  survey  of  a  vast  field  of  biology  into  which 
we  are  led  by  the  activities  of  the  simple  organism  Yeast,  we 
learn  that  vital  activities  of  animals  and  plants  consist  of  a  series 
of  intricate  reactions  through  the  agency  of  enzymes  and  other 
subtle  chemical  bodies.  Partly  by  experiment  in  the  hands 
of  eminent  physiologists  like  Hoppe-Seyler,  Ostwald,  Hof- 
mei'ster,  Mathews  and  others,  and  partly  by  theory,  we  are 
brought  to  the  present-day  conclusion  that  not  only  cleavage 
processes  which  occur  during  the  preparation  for  assimilation 
of  foods,  not  only  all  of  the  processes  of  destructive  metabolism 
with  the  consequent  liberation  of  energy  in  the  form  of  light,  heat, 
electricity,  or  movement,  but  also  all  of  the  synthetic  processes 
in  protein  formation,  from  the  union  of  the  simplest  beginning 
materials  like  carbonic  acid,  water,  and  nitrogen-holding  salts 
to  the  most  complicated  albumen  compounds,  are,  one  and  all, 
effects  of  specific  chemical  bodies,  each  of  which  plays  its  part. 


CHAPTER  III 

ORGANISMS  OF  ONE  CELL.     Continued 
THE  UNICELLULAR  ANIMALS 

IN  cells  of  tissues  in  higher  animals  some  one  vital  function 
predominates  over  all  others  and  gives  to  the  cell  its  particular 
character.  With  muscle  cells,  the  function  of  contractility  or 
movement  overshadows  all  others;  with  nerve  cells,  irritability 
or  nervous  response  to  external  stimuli  is  predominant;  with 
epithelial  cells,  the  function  of  secretion  predominates,  and  so 
on,  each  type  of  cell  performing  its  own  work  but  all  working 
for  the  good  of  the  organism  as  a  whole.  Some  of  the  vital 
functions  indeed  are  lost  with  such  differentiation ;  the  function 
of  digestion,  for  example,  is  confined  to  cells  of  the  alimentary 
tract  while  the  latter  have  lost  all  power  of  independent  move- 
ment. Such  cells  in  which  one  function  overrides  all  others  may 
be  spoken  of  as  physiologically  unbalanced  cells,  while  other  cells 
in  which  the  functions  or  vital  activities  are  equally  developed 
may  be  spoken  of  as  physiologically  balanced.  Such  balanced 
cells  are  illustrated  by  the  entire  group  of  protozoa  or  animals 
consisting  of  one  cell  only,  a  group  comprising  some  of  the  ni'  st 
perfect  of  single  cells  and  at  the  same  time  the  simplest  in 
structure  of  all  animals. 

A.  AMOEBA  PROTEUS 

Few  organisms  have  been  studied  more  frequently  than  this 
minute  protozoon,  and  nothing  gives  a  better  idea  of  living 
matter  than  this  fascinating  bit  of  protoplasm  with  its  enig- 
matical movement  and  its  vital  activities.  It  is  found  in  the 
greatest  variety  of  places  but  is  not  as  plentiful  as  many  text 
books  would  lead  one  to  suppose.  It  may  be  found,  however, 
among  the  superficial  dead  leaves  and  slime  in  many  ponds  or 
4  44 


AMOEBA  PROTEUS 


45 


on  the  stems  of  ordinary  water  plants  where  amoebae  and 
allied  forms  are  often  abundant.  Amoeba  proteus  varies  in  size 
from  Mooth  to  Mstn  of  an  mcn  an(^  undergoes  different  form 
changes  which  at  times  make  it  difficult  to  recognize.  There 
is  some  question  whether  some  of  the  forms  described  as  dif- 
ferent species  of  amoeba  are  not  in  reality  one  and  the  same; 
the  matter  can  be  decided  only  by  knowledge  of  the  complete 


FIG.  18. — Amoeba  proteus  in  active  moving  condition,  c.v.,  Contractile 
vacuole;  f.v.,  food  vacuole;  n,  nucleus;  p,  remains  of  former  pseudopodia;  w.v., 
water  vacuoles.  The  arrows  indicate  the  direction  of  protoplasmic  flow.  (From 
Sedgwick  and  Wilson.) 


life  history.  Sometimes  the  organism  is  flattened  or  spatulate 
in  form,  changing  slowly,  if  at  all,  in  shape.  Again  it  becomes 
a  quickly  moving  drop  of  protoplasm  quite  transparent  and 
long  drawn  out  as  a  single  thread  of  substance.  All  interme- 
diate grades  between  these  forms  are  known,  and  in  a  general 
wav  the  form  and  movements  indicate  states  of  nutrition,  for 
the  flat  spatulate  types  are  usually  dense  with  undigested  food 


46  ORGANISMS  OF  ONE  CELL 

while  the  rapidly  moving  forms  are  relatively  clear  and  trans- 
parent (Fig.  1 8). 

Nucleus. — While  the  form  of  Amoeba  proteus  is  continually 
changing,  the  structure  remains  practically  the  same.  It  is 
always  one  protoplasmic  unit,  a  cell,  and  like  other  cells  it  is 
differentiated  into  cell  body,  called  cytoplasm,  and  nucleus. 
This  nucleus  is  composed  of  slightly  different  protoplasm  from 
that  of  the  cell  body,  and  is  chiefly  characterized  by  the  greater 
affinity  for  basic  stains,  while  the  protoplasm  of  the  cell  body 
has  an  equivalent  affinity  for  acid  stains.  The  substance  of 
the  nucleus  which  takes  the  stain  is  called  chromatin,  and  is 
composed  chiefly  of  nucleins  or  nucleo-proteins  particularly 
rich  in  phosphorus.  The  nucleus  can  be  seen  in  life  as  a  pale 
circular  disc  moving  with  the  flow  of  granules  from  one  part  of 
the  cell  to  another,  and  turning  as  it  moves,  showing  now  the 
circular  outline,  again  the  flattened  edge  view. 

Endoplasm  and  Ectoplasm. — The  cytoplasm  is  not  entirely 
homogeneous  but  is  'differentiated  into  an  inner  and  an  outer 
portion.  The  former,  in  which  the  nucleus  is  found  is  called 
the  endoplasm,  or  sometimes,  the  endosarc.  The  latter,  called 
the  ectoplasm  or  ectosarc,  although  soft  and  gelatinous,  is  firmer 
and  denser  than  the  endoplasm  and  is  more  transparent,  for 
it  has  none  of  the  refractive  bodies  found  in  the  endoplasm.  It 
is  to  be  regarded  as  a  protective  layer,  since  it  is  the  part  that 
comes  in  contact  with  the  surrounding  medium,  and  through 
it  all  intercourse  between  the  amoeba  and  the  environment 
must  take  place.  In  other  forms  of  protozoa  it  is  this  ecto- 
plasm which  becomes  differentiated  in  the  greatest  variety  of 
ways  for  purposes  of  protection,  locomotion,  and  sensation. 

The  endoplasm,  on  the  other  hand,  contains  the  vital  organs 
of  the  cell,  a  number  of  which  can  be  seen  with  little  effort. 
Large  particles  more  or  less  disintegrated,  and  surrounded  by 
clear  fluid,  are  food  bodies  recently  ingested  and  are  undergoing 
digestion  in  the  fluid-filled  spaces,  which,  for  this  reason,  are 
called  gastric  vacuoles.  The  bodies  in  these  vacuoles  may  fre- 
quently be  recognized  as  portions  of  other  minute  animals  or 
plants.  Another  fluid-filled  sphere,  usually  best  seen  near  the 


FOOD-TAKING  BY  AMOEBA  47 

periphery  of  the  cell,  is  perfectly  clear  and  without  solid  particles 
of  any  kind.  It  suddenly  disappears,  the  contained  fluid  being 
excreted  to  the  outside  through  the  ectoplasmic  layer.  It 
shortly  reappears  as  a  small  clear  space  which  rapidly  grows  in 
size  until  it  again  disappears  by  contraction.  From  its  rhythmic 
dilatation  and  contraction  this  organ  of  the  cell  is  called  the 
contractile  vacuole;  its  pulsations  are  fairly  constant  and  regu- 
lar in  the  same  temperature  but  the  rate  varies  with  changes  in 
temperature.  This  organ  was  formerly  believed  to  be  a  beat- 
ing heart,  but  is  now  generally  regarded  as  an  excretory  or 
possibly  respiratory  center. 

Movement. — In  the  course  of  its  "amoeboid  movement" 
Amoeba  throws  out  blunt  protoplasmic  processes  termed 
pseudopodia.  The  ectoplasm  gives  way  at  one  point  as  a 
result  of  unknown  inner  forces ;  the  endoplasmic  granules  may 
be  seen  streaming  toward  this  point  until  a  blunt  finger-formed 
pseudopodium  results.  Sometimes  the  entire  mass  of  the 
Amoeba  flows  through  this  extended  portion,  and  the  organism 
then  will  have  moved  a  distance  equal  to  its  diameter.  In  the 
meantime,  however,  other  pseudopodia  may  be  forming  and 
the  direction  of  movement  changed.  As  the  cell  changes  in 
moving  direction  the  older  pseudopodia  are  withdrawn,  and 
these  may  be  seen  as  small  blunt  processes  of  the  cell  on  the 
side  opposite  that  in  advance  (cf.  Fig.  10). 

Metabolism. — The  constant  streaming  of  protoplasm  and  the 
constant  formation  of  pseudopodia  require  energy.  The  his- 
tory of  energy  transformation  resulting  in  the  advance  of  an 
Amoeba  involves  the  entire  history  of  metabolism  and,  if 
understood,  would  make  the  matter  of  "vital  activities"  an 
open  secret.  Some  few  points,  however,  are  known.  The 
immediate  source  of  energy  for  such  movements  is  the  com- 
bustion or  oxidation,  through  the  action  of  enzymes,  of  parts 
of  the  cellular  protoplasm,  and  if  no  food  were  taken  in,  the 
organism  would  soon  die  from  loss  of  its  vital  parts.  This  loss, 
however,  is  constantly  made  good  by  capture  and  digestion  of 
food,  functions  involving  the  primary  and  fundamental  activi- 
ties of  all  animals  and  plants,  these  functions  being  the  central 


'48 


ORGANISMS  OF  ONE  CELL 


factors  in  the  development  of  the  multifold  structures  in  the 
living  world. 

Amoeba  proteus  captures  food  and  draws  it  into  the  body 
protoplasm  through  the  agency  of  the  pseudopodia,  or  rather, 
the  protoplasm  of  the  organism  streams  into  pseudopodia 
around  the  prey,  thus  engulfing  it  (Fig.  19).  Some  water  is 
engulfed  with  the  food  particle  and  this  forms  the  chief  portion 
of  the  liquid  of  the  gastric  vacuole.  This  water  usually  is 
alkaline  in  reaction  when  taken  into  the  body;  soon,  however, 


FIG.  19. — Amoeba  dividing  (A},  ingesting  food,  and  encysted  (D).  p, 
Retracted  pseudopodium;  dt,  diatom  taken  in  as  food;  other  letters  as  in  Fig.  18. 
(From  Sedgwick  and  Wilson  after  Leidy  and  Howes.) 

it  changes  in  reaction  from  alkaline  to  acid,  the  change  being 
brought  about  by  the  secretion  of  an  acid  digestive  fluid  from 
the  surrounding  endoplasm.  Through  the  action  of  this 
acid  (supposed  to  be  HC1),  the  food  particle,  if  living,  is  first 
killed  and  then  disintegrated. l  Later  the  reaction  of  the  con- 
tents of  the  vacuole  again  changes  from  acid  back  to  alkaline, 
and  in  this  medium  the  further  processes  of  digestion  are  accom- 
plished. The  end  result  of  this  series  of  chemical  actions  on 
the  food  is  the  formation  of  proteoses  from  the  protein  sub- 

1  No  enzyme  analogous  to  pepsin  and  acting  in  this  acid  medium  is  known  to 
occur  in  Amoeba  proteus. 


AMOEBA  PROTEUS  49 

stances  which,  as  soluble  materials,  are  then  taken  into  the 
body  protoplasm  of  the  organism ;  the  food  particles  are  said 
to  be  digested. 

When  the  protein  food  is  thus  digested  it  is  only  prepared  for 
the  first  stages  of  protoplasm  re-building,  and  many  more  steps 
must  be  taken  before  the  essential  elements  are  added  to  the 
protoplasmic  molecules.  These  further  steps  are  generally 
included  under  the  comprehensive  term,  assimilation,  and  their 
exact  nature  is  hidden  in  the  deepest  obscurity.  Little  by  little 
chemical  research  is  throwing  light  on  the  processes,  so  that  we 
have  now  a  basis  for  working  hypotheses  as  to  the  manner  in 
which  protoplasmic  molecules  are  built  up.  We  now  know  that 
cells  in  all  kinds  of  tissue  possess  chemical  properties  hitherto 
unsuspected.  These  have  to  do,  in  the  main,  with  enzymes 
which  act  upon  the  partially  broken  down  food  matters  and 
cause  their  disintegration  into  finer  particles,  until  they  are 
prepared  for  anchorage  in  the  protoplasmic  molecules,  and  be- 
gin the  series  of  integrations  and  disintegrations  characteristic 
of  vital  processes.  It  has  been  found  by  experiment  that 
portions  of  the  cell  without  a  nucleus  cannot  form  such  enzymes, 
and  the  conclusion  is  drawn  that  these  vital  activities  are  de- 
pendent upon  substances  derived  from  the  chromatin.  It  was 
largely  through  experiments  in  cutting  minute  forms  like 
Amoeba  that  this  discovery  was  made.  Nusbaum,  Hofer, 
Verworn  and  others  found  that  if  an  amoeba  is  cut  in  two  pieces 
with  a  scalpel,  both  parts  would  continue  to  live  for  some  days 
but  one  would  die  ultimately,  while  the  other  part,  containing 
a  nucleus,  would  continue  to  live  and  multiply  indefinitely. 
The  portion  without  a  nucleus  is  unable  to  digest  and  assimi- 
late food,  or  even  to  capture  it. 

Thus  while  the  exact  processes  of  assimilation  are  unknown 
in  amoeba,  it  is  quite  probable  that  the  finer  changes  are  carried 
on  in  the  same  way  as  with  cells  in  higher  animals,  that  is, 
through  the  agency  of  enzymes.  These  act  in  a  linked  series, 
the  product  of  one  chemical  action  furnishing  the  material  for 


50  ORGANISMS  OF  ONE  CELL 

the  next  until  finally  the  elements  derived  from  the  protein  food 
are  added  to  the  protoplasmic  molecules. 

Knowledge  of  the  anabolic  or  building  processes  is  much  more 
hypothetical  than  that  in  regard  to  the  katabolic  or  breaking 
down  processes,  the  latter  taking  place  whenever  energy  is 
expended.  These  processes  take  place  in  the  cell  body  or  cyto- 
plasm and  are  due  to  enzymes  which  probably  come  from  the 
inter-action  of  cell  nucleus  and  cytoplasm.  They  are  oxidizing 
enzymes  which  bring  about  the  union  of  oxygen  with  receptors 
of  the  protoplasmic  molecules.  The  ultimate  result  of  such 
combustion  is  the  formation  of  simple  compounds,  the  hydrogen 
leaving  the  protoplasm  molecule  to  unite  with  the  oxygen,  form- 
ing water ;  carbon  with  oxygen,  forming  carbon  dioxide,  and  the 
ammonium  combination  (NHs),  forming  with  carbon  and  oxy- 
gen the  compound  urea  (NH^CO,  which  still  contains  some 
energy.  This  urea,  however,  cannot  be  used  by  the  animal  pro- 
toplasm as  a  further  source  of  energy  but  is  voided  to  the  outside 
as  waste  matter.  The  energy,  however,  is  not  wasted  in  nature 
for,  as  we  have  seen,  bacteria  have  the  power  of  breaking  urea 
into  free  ammonia,  carbon  dioxide  and  water,  and  of  converting 
the  contained  energy  into  the  energy  of  their  own  vital 
processes. 

As  urea  is  of  no  use  to  the  animal  but  rather  a  menace  in  case 
of  its  undue  accumulation,  it  is  necessary  for  the  animal  to  get 
rid  of  it.  In  all  animals  this  is  accomplished  by  means  of 
special  organs  which  form  the  excretory  systems.  In  mammals 
and  vertebrates  generally,  the  kidney  and  associated  organs 
are  set  apart  for  this  purpose;  in  lower  animals  like  worms, 
Crustacea,  etc.,  special  funnel-like  organs  termed  "nephridia" 
perform  this  function.  In  all  types  of  animals  in  short,  there 
is  some  structure  or  structures  of  more  or  less  complicated  type 
which  are  devoted  almost  exclusively  to  this  end. 

In  Amoeba  proteus  there  is  a  special  organ  which  has  the  func- 
tion of  disposing  of  waste  matters  and  is  analogous,  therefore, 
to  the  kidney  of  higher  types.  This  is  the  "  contractile  vacuole" 
which  pulsates  with  a  regular  or  rhythmic  contraction,  the 
rate  of  pulsation  varying  with  the  temperature.  It  is  not  im- 


AMOEBA  PROTEUS  51 

probable  that  the  contractile  vacuole  has  other  functions  than 
that  of  urea  excretion,  but  this  has  never  been  demonstrated, 
while  the  presence  of  uric  acid  crystals  has  been  shown  in  the 
fluids  of  the  vacuole.  After  a  vacuole  has  contracted,  the  new 
one  is  formed  in  the  vicinity  of  the  nucleus  by  union  of  small  and 
at  first  unnoticeable  vesicles,  one  or  more  of  which  may  be  left 
over  by  the  incomplete  emptying  of  the  preceding  one.  This 
coalescence  continues  until  the  new  vacuole  becomes  large 
enough  to  be  seen  with  low  magnification.  As  it  grows  by  the 
continual  addition  of  fluids  it  increases  in  bulk  and  is  less  easily 
carried  in  the  moving  protoplasm,  so  that  it  becomes  left  behind, 
so  to  speak,  until  it  bursts  to  the  outside,  usually  in  that  region 
which  for  the  time  being  is  posterior. 

Irritability. — As  a  spark  may  cause  an  explosion  so  may  cer- 
tain agents  in  the  environment  produce  sudden  and  violent 
reactions  on  the  part  of  a  living  organism.  Such  reactions  are 
due  to  the  local  expenditure  of  energy.  In  higher  animals 
special  " sensory"  organs  are  activated  by  heat,  light,  sound, 
electrical  or  other  agents  called  stimuli.  The  energy  released  as 
a  response  to  such  stimuli  is  far  in  excess  of  the  energy  repre- 
sented by  the  activating  agents,  and  may  involve  reactions  by 
every  part  of  the  organism. 

In  Amoeba  proteus  there  are  no  sense  organs  but  the  organism 
has  the  property  of  reacting  to  every  marked  change  in  external 
conditions  in  its  environment.  This  property  is  called  irrita- 
bility, and  is  analogous  to  more  complicated  reactions  to  stimuli 
in  higher  animals.  Innumerable  kinds  of  stimuli  may  produce 
these  reactions  but  may  be  classified  according  to  their  qualities 
into  a  few  large  groups,  such  as  mechanical,  chemical,  photic  and 
electrical,  all  of  which  indicate  changes  in  the  immediate  en- 
vironment of  the  organism.  The  responses  of  organisms  to  the 
great  variety  of  stimuli  are  so  diverse  that  only  the  most  general 
definition  will  cover  them  all.  The  physiologist  Verworn  gives 
such  a  definition  as  follows:  " Irritability  of  living  substance  is 
its  capacity  of  reacting  to  changes  in  its  environment  by  changes 
in  the  equilibrium  of  its  matter  and  its  energy"  (Lee,  Transla- 
tion, p.  353). 


52  ORGANISMS  OF  ONE  CELL 

With  Amoeba  proteus  irritability  is  indicated  by  more  rapid 
movement,  as  under  the  stimulation  of  increased  temperature; 
or  by  withdrawing  of  pseudopodia  and  rounding  out  of  the  body, 
as  under  the  effects  of  mechanical,  electrical  or  chemical  stimuli. 
These  various  responses  frequently  subserve  a  useful  purpose  in 
capturing  food  or  avoiding  difficulties,  and  represent  a  proto- 
type of  higher  conscious  actions.  There  is  absolutely  no  ground 
for  believing  that  Amoeba  does  anything  intentionally  or  wil- 
fully but  all  of  its  activities  can  be  explained  on  the  ground  of  re- 
sponses to  environmental  stimuli.  An  interesting  analogy  to 
vital  processes  in  Amoeba  is  shown  by  .a  drop  of  chloroform 
which,  through  surface  tension,  will  draw  in,  and  roll  up,  a 
filament  of  shellac.  Another  species  of  Amoeba,  A.  verrucosa, 
captures  and  rolls  up  a  filament  of  Oscillaria  in  exactly  the 
same  way,  and  the  inference  is  that  both  processes  are  due  to 
the  same  fundamental  physical  laws. 

Reproduction. — In  all  forms  of  life  the  process  of  reproduction 
when  reduced  to  its  simplest  terms  is  the  same,  viz.  cell  division, 
and  the  young  forms  invariably  begin  life  as  single  cells  which, 
after  the  stimulus  of  fertilization  or  its  equivalent,  begin  to 
divide.  The  products  of  this  continued  division  soon  begin  to 
differentiate  into  tissues  and  organs,  the  various  phenomena 
constituting  the  subject  matter  of  the  science  Embryology. 
In  Amoeba  proteus,  however,  the  cell  is  never  more  than  a  single 
unit  and  might  at  all  times  be  considered  the  equivalent  of  an 
egg.  There  is  evidence,  although  proof  is  not  certain,  that  only 
at  definite  periods  is  Amoeba  really  similar  to  an  egg  cell,  and 
requiring  fertilization  for  its  continued  activities.  At  other 
times  the  cell  divides  as  does  the  fertilized  egg  of  other  animals, 
but,  unlike  the  products  of  cleavage  of  the  egg,  the  daughter  cells 
of  Amoeba  do  not  remain  attached  to  one  another  but  separate 
and  live  as  independent  organisms  similar  to  the  parent.  Here 
then,  as  with  the  yeast  cell,  reproduction  is  reduced  to  its  lowest 
terms,  simple  division.  In  this  process  the  nucleus  of  the  cell 
first  divides  and  then  the  cell  body  (Figs.  19  and  20). 

Encystment. — When  the  environmental  conditions  become 
unsuitable  for  life,  an  Amoeba  will  secrete  about  itself  a  wall  or 


FLAGELLATED  PROTOZOA  53 

cyst  of  chitin,  within  which  it  is  protected  against  adverse 
conditions  such  as  drought.  When  conditions  are  again 
suitable  the  cyst  wall  is  ruptured,  and  the  organism  comes 
out  for  a  new  cycle  of  growth  and  reproduction. 

B.  FLAGELLATED  PROTOZOA.     CHILOMONAS  AND  ALLIED 

FORMS 

When  protein  matter,  a  piece  of  beef  or  vegetable,  is  left  in 
water  for  a  day  or  so  it  disintegrates  and  putrefies  under  the 
action  of  bacteria.  In  addition  to  the  swarms  of  these  minute 


FIG.  20. — Entamoebae  from  the  intestine  during  division  stages  of  the  nucleus. 

From  preparations. 

bacteria  there  may  also  appear  in  the  water  enormous  numbers  of 
small  flagellated  protozoa,  Chilomonas  par amecium.  Compared 
with  the  smaller  bacteria  these  minute  animals  are  of  consider- 
able size,  but  compared  with  ^4  moeba  proteus  they  are  quite  small, 
having  a  length  of  about  25  to  30^  (Kooo  to  J^QO  of  an  inch.) 
In  form  they  are  somewhat  like  an  elongated  foot-ball  with  an 
obliquely  truncated  end,  which  we  may  term  the  anterior  end 
since  this  is  the  end  in  advance  when  swimming  (Fig.  21). 
The  posterior  end  is  rounded  and  blunt,  and  has  no  structural 
features  of  importance.  The  animal  moves  through  the  water 
by  means  of  two  parallel  flagella  which  extend  out  to  a 
distance  equal  to  the  total  length  of  the  body,  the  latter  being 


54 


ORGANISMS  OF  ONE  CELL 


dragged  along  by  the  vigorous  lashing  of  the  water  by  these 
flagella,  and  turning  round  and  round  on  its  long  axis  as  it 
moves  forward. 

It  is  because  of  the  flagella  that  Chilomonas  is  classified  as 
one  of  the  Mastigophora  or  whip-bearing  protozoa. 

The  flagella  are  extremely  delicate,  and  impossible  to  see  when 
the  organism  is  moving,  but  when  the  cells  are  killed  with  iodine 
they  can  be  made  out  easily.  They  are  of  uniform  diameter 


FIG.  21. — Flagellated  protozoa,  Chilomonas,  Peranema,  and  Euglena.  A, 
Chilomonas  paramecium',  B,  Peranema  trichophora  at  beginning  of  division,  i, 
Flagella;  2,  basal  bodies  of  flagella;  3,  basal  body  with  young  flagellum  growing 
from  it;  4,  parabasal  body;  5,  nuclei.  Drawings  and  photograph  from  prepara- 
tions. 


throughout,  entering  the  body  at  about  the  center  of  the 
truncated  plane  and  continuing  into  the  protoplasm  as  far  as 
the  nucleus.  The  latter  has  a  different  structure  from  the 
nucleus  of  Amoeba  proteus,  and  consists  of  a  relatively  large 
granule  (division  center)  surrounded  by  minute  granules  of 
chromatin,  and  with  a  delicate  nuclear  membrane.  Between 
the  nucleus  and  the  truncated  end  of  the  cell  is  a  somewhat 
cone-shaped  mass  of  denser  protoplasm  which  is  probably  the 
main  seat  of  food  assimilation.  The  remaining  protoplasm  has 
a  distinct  alveolar  structure,  the  alveoli  about  the  periphery 


FLAGELLATED  PROTOZOA  55 

being  much  more  regular  and  compact  than  those  within,  the 
whole  giving  a  very  excellent  demonstration  of  the  finer  struc- 
ture of  protozoan  protoplasm.  A  contractile  vacuole,  finally, 
can  be  seen  at  one  side  of  the  truncated  end. 

Chilomonas  differs  from  Amoeba  structurally  in  having  a 
definite  and  constant  body  form  due  to  the  presence  of  a  firm  cell 
membrane,  easily  seen  in  stained  specimens.  It  also  differs 
from  Amoeba  proteus  and  from  the  majority  of  animals  physio- 
logically in  that  no  solid  food  is  taken  in  to  be  digested,  as  in  a 
gastric  vacuole  of  Amoeba.  Nevertheless  it  could  not  live 
without  protein  food  in  some  form,  and  the  fact  that  it  does  live 
and  multiply  to  enormous  numbers  shows  that  it  obtains  suit- 
able food.  This  it  gets  from  the  relatively  small  amount  of 
protein  matter  coming  from  the  meat  or  vegetable  that  is  dis- 
solving in  the  water  and  absorbed  by  osmosis  into  the  proto- 
plasm, the  chief  area  of  absorption  being  the  truncated  end. 
This  method  of  feeding,  widely  distributed  among  similar 
lower  forms  of  life,  is  called  saprophytic  or  saprozoic  nutrition, 
while  that  of  Amoeba  and  higher  animal  forms  is  called  holo- 
zoic  nutrition.  Many  plants  like  bacteria  and  fungi  take  food 
in  a  similar  way,  the  process  being  called  saprophytic  nutrition, 
while  in  typical  green  plants,  where  the  methods  of  feeding  are 
entirely  different,  the  term  holophytic  nutrition  is  employed. 
While  the  latter  method  of  feeding  (to  be  explained  in  detail 
in  connection  with  the  fern)  is  almost  exclusively  confined  to 
the  plant  kingdom,  there  are  some  few  animals,  as,  for  example, 
Euglena  and  its  allies,  which  may  make  their  food  by  the  plant 
method. 

We  are  ignorant  of  the  finer  processes  of  assimilation  in  Chilo- 
monas but  are  justified  in  assuming  that  it  differs  in  no  essential 
respect  from  the  processes  in  Amoeba,  after  the  protein  food 
materials  are  dissolved.  As  there  are  no  solids  the  cell  contains 
no  undigested  food  matters,  and  there  is  no  defecation.  Urea, 
however,  is  undoubtedly  formed  since  the  organism  is  con- 
stantly doing  work,  and  this  is  probably  excreted  by  means 
of  the  contractile  vacuole,  although  it  may  also  pass  out  by  exos- 


56 


ORGANISMS  OF  ONE  CELL 


mosis,  as  must  be  the  case  in  many  protozoa  in  which  no 
contractile  vacuole  can  be  found. 

Chilomonas  may  frequently  be  seen  in  pairs  swimming 
along  side  by  side;  these  are  sister  cells  not  yet  fully  divided 
and  they  have  originated  by  the  longitudinal  division  of  the 
cell.  Reproduction  thus  is  of  the  simplest  type,  the  nucleus 
always  dividing  first,  then  the  cell  body,  while  new  flagella 


FIG.  22. — A  flagellated  protozoon,  Copromonas  subtilis.  A,  A  normal  adult  cell 
before  division;  B,  two  individuals  in  conjugation;  C,  D,  E,  and  F,  later  stages  in 
the  fusion  of  cells  and  nuclei,  and  formation  of  protective  cyst.  (From  Dobell.) 

are  formed  as  outgrowths.  It  is  not  known  how  these  originate 
in  Chilomonas  but  from  analogy  with  other  forms  of  flagellates 
where  the  process  is  known,  two  of  the  four  at  least  must  be 
new  growths ;  in  some  cases  the  old  flagella  are  withdrawn  and 
new  ones  are  formed;  in  other  cases  one  of  the  two  flagella  goes 
to  each  daughter  cell. 


FLAGELLATED  PROTOZOA 


57 


The  processes  of  feeding,  growing  and  dividing  continue  for 
days  before  other  activities  occur.  Indeed  for  Chilomonas  so 
far  as  known  they  may  continue  indefinitely,  but  from  analogy 
with  other  forms  of  Mastigophora  where  the  full  life  history  is 
known,  these  ordinary  vegetative  processes  are  sooner  or  later 
replaced  by  processes  involving  a  simple  kind  of  fertilization 
or  sexual  union.  In  Copromonas,  for  example,  two  similar 
cells  after  a  long  period  of  divisions,  meet  and  fuse;  the  flagellum 
of  one  of  them  is  discarded  while  that  of  the  other  is  used  as  a 
motile  apparatus  for  the  pair. 
Fusion  of  nucleus  and  cell  body 
continue  until  a  single  cell  results. 
This  then  secretes  a  membrane  and 
becomes  quiescent,  or  it  divides  and 
behaves  like  an  ordinary  individual. 
Here  there  is  typical  fertilization  but 
no  difference  between  the  conjugat- 
ing cells  so  far  as  can  be  detected 
(Fig.  22). 

Allied  Forms. — Many  hundreds  of       FIG.   ^.—Synura  uvella,  a 

colony    of     flagellated    proto- 
species    Of    flagellated    protozoa    are    zoa  in   which   the   individuals 

known  and  may  exhibit  the  most  are  attached 
manifold  variations  in  structures 
and  functions.  Many  of  them  have  only  one  flagellum, 
as  Peranema  or  Euglena,  for  example,  which  are  common 
organisms  in  infusions  of  different  kinds.  It  is  a  remarkable 
and  fascinating  sight  to  see  a  relatively  large  cell  like  Pera- 
nema drawn  steadily  forward  by  the  undulations  of  the  tip  of 
its  long  and  easily  seen  flagellum.  In  this  case  the  entire 
flagellum  does  not  vibrate,  but  only  the  tip,  whereas  in  Euglena 
the  whole  flagellum  is  in  constant  motion  and  almost  invisible. 
Nutrition  in  Peranema,  as  in  Chilomonas,  is  saprozoic,  but  it 
is  entirely  different  in  the  case  of  Euglena  which  has  the  power 
to  manufacture  its  food  in  the  same  way  that  the  higher  green 
plants  do.  This  holophytic  nutrition  is  accomplished  through 
the  agency  of  chloroplastids  or  color-bearing  structures  distrib- 
uted throughout  the  protoplasm  of  the  Euglena  cell  (Fig.  21). 


at    a    common 
From  a  photograph. 


58  ORGANISMS  OF  ONE  CELL 

The  color  is  due  to  a  substance  termed  chlorophyll  which,  with 
the  aid  of  sunlight,  is  able  to  manufacture  starch;  this,  in  turn, 
is  built  up  into  protein  matter  which  serves  as  food  (see  Chapter 
V).  Another  colored  structure  is  also  found  in  Euglena, 
although  it  is  not  so  conspicuous  as  the  green  chloroplastids. 
This  is  the  red-colored  spot  or  stigma  which  is  more  sensitive 
to  light  than  other  parts  of  the  protoplasm,  and  is  often  spoken 
of  as  a  rudimentary  "  eye-spot."  In  many  cases  it  is  accom- 
panied by  a  lens-like  body  which  may  concentrate  light  rays 


FIG.  24. — Uroglena  americana,  a  colony  of  flagellated  protozoa  in  which  the  indi- 
viduals are  embedded  in  a  common  gelatinous  matrix. 

on  a  particular  spot  and  so  act  as  a  directive  agent;  at  any 
rate  this  spot  is  usually  turned  toward  the  source  of  light 
and  it  serves  therefore  as  a  rudimentary  sense  organ. 

Colonies. — Both  Peranema  and  Euglena  reproduce  by  longi- 
tudinal division  which  is  not  different  in  any  way  from  the  divi- 
sion of  Chilomonas.  The  daughter  cells  separate  after  division, 
and  lead  an  independent  existence.  In  some  forms  of  flagel- 
lated protozoa,  however,  the  cells  after  division  do  not  separate 
completely  but  remain  attached  to  each  other  in  one  way  or 
another  (e.g.,  by  the  basal  ends  as  in  Synura  uvella  (Fig.  23),  thus 
forming  aggregates  of  cells  or  individuals  of  a  second  order  to 
which  the  term  colony  is  given.  Sometimes  the  cells  thus 
formed  are  embedded  in  a  common  jelly,  the  aggregate  form- 
ing relatively  large  spherical  masses  (Fig.  24).  Again  they  are 


FLAGELLATED  PROTOZOA  59 

limited  to  a  certain  number  of  cells,  and  this  number  always 
reappears  upon  reproduction  so  that  the  multicellular  individual 
is  much  more  specific  in  nature,  as  in  Goriium  pectorale  where 
the  individual  always  consists  of  16  cells  (Fig.  25). 

These  colony  forms  are  of  peculiar  interest  in  that  they  have 
many  features  in  common  with  the  higher  animals  and  plants, 


FIG.  25. — Gonium  pectorale,  a  colony  of  flagellated  protozoa  consisting  of  sixteen 
cells  arranged  in  a  flat  plate,  three  on  a  side  and  four  in  the  center.  Each  cell 
carries  two  flagella.  Photograph  frorn  a  preparation. 

but  the  cells  are  not  differentiated  for  the  performance  of  differ- 
ent functions,  each  one  acting  for  itself  rather  than  for  the  ag- 
gregate as  a  whole.  They  represent,  therefore,  a  phase  in 
complexity  of  form  and  function  intermediate  between  the  uni- 
cellular organisms  (protozoa,  protophyta)  and  the  multicellular 
(metazoa  and  metaphyta). 


60 


ORGANISMS  OF  ONE  CELL 


TWCHOCY8TS 


5"  CANALS 


C.  A  CILIATED  PROTOZOON,  PARAMECIUM  CAUDATUM 

An  infusion  of  vegetable  or  animal  matter  becomes  the  feed- 
ing ground  not  only  of  bacteria  and  flagellated  protozoa,  but 
also,  after  some  considerable  time,  of  ciliated  protozoa  as  well. 
From  the  fact  that  all  of  these  organisms  appear  in  such  infu- 
sions, the  term  Infusoria 
was  formerly  employed  to 
designate  all  of  them  indis- 
criminately. The  bacteria 
were  first  recognized  as 
having  no  systematic  rela- 
tion to  the  other  forms, 
and  were  separated  in 
classification  from  the  pro- 
tozoa found  in  infusions. 
Later  the  flagellated  forms 
were  recognized  as  entirely 
different  from  the  ciliated 
ones  and  were  classified 
under  the  name  Mastigo- 
phora,  so  that  finally,  the 
term  Infusoria  is  used 
today  to  include  only  the 
ciliated  forms  of  protozoa. 
Of  these  there  is  a  great 
number  of  different  types, 
one  of  which,  Paramecium, 
formerly  known  as  the 
"slipper  animalcule"  com- 
mon in  every  ditch,  pool, 
or  stagnant  water,  may 
serve  as  a  type  for  all. 

Paramecium  is  an  elon- 
gated, somewhat  cigar-shaped  organism,  consisting  of  a  single 
cell  which  moves  rapidly  through  the  water,  turning  the  while 
on  its  long  axis.  The  movement  is  brought  about  by  the 


pSRlSTOMS 
MOUTH  AAIO  GULLET 


CONTRACTILE 


FIG.  26. — Diagram  of  structures  of  Para- 
mecium caudatum  from  an  individual  about 
°f  an  incn  *n  length. 


PARAMECIUM  CAUDATUM  61 

synchronous  beating  of  the  water  by  myriads  of  minute  cilia 
uniformly  distributed  over  the  surface  in  diagonal  lines,  while 
the  rotation  on  the  long  axis  is  due  partly  to  this  arrangement 
and  partly  to  the  action  of  cilia  covering  an  asymmetrical 
groove  called  the  peristome,  extending  from  the  anterior  end 
backward  to  about  the  middle  of  the  body.  This  peristomial 
area  does  not  run  in  a  straight  line  from  the  anterior  end  to 
the  center  but  curves  from  the  left  dorsal  extremity  to  the 
right,  ending  to  the  left  of  the  middle  of  the  ventral  surface 
at  the  mouth.  The  area  deepens  from  in  front  backward,  until 
at  the  mouth  a  conspicuous  pocket  is  formed  (Fig.  26).  The 
mouth  is  a  circular  opening  leading  into  a  short  gullet  which 
bears  an  undulating  membrane  on  one  side. 

Structure. — The  finer  structure  of  Paramecium  differs  con- 
siderably from  that  of  Amoeba  and  Chilomonas  owing  to 
the  fact  that  Paramecium  cells  are  much  more  highly  differen- 
tiated into  cellular  organs.  We  can  recognize,  however,  a 
distinct  endoplasm  and  ectoplasm  and  note  that  the  chief 
differentiations  are  in  the  latter.  The  endoplasm  is  made  up 
of  alveoli  similar  to  those  of  Amoeba,  and  we  find  the  same 
granular  microsomes  and  larger  food  particles  in  various  stages 
of  digestion.  The  protoplasm  also  undergoes  streaming  move- 
ments or  cyclosis,  the  movement  being  entirely  within  the  cell, 
however,  and  irregular  so  that  it  appears  to  be  different  from 
the  movement  involving  pseudopodia  formation  in  Amoeba. 
Nuclei  and  contractile  vacuoles  are  quite  different  and  more 
complex  than  in  Amoeba  or  Chilomonas. 

Nuclei. — Paramecium  and  the  Infusoria,  in  general,  are  differ- 
ent from  all  other  cells  in  having  two  kinds  of  nuclei,  macronu- 
clei  and  micronuclei.  One  of  these,  the  macronucleus,  is  large 
and  conspicuous;  the  other,  the  micronucleus,  is  very  small  and 
usually  partly  embedded  in  the  substance  of  the  larger  nucleus. 
While  not  fully  proved  it  is  probable  that  these  two  kinds  of 
nuclei  have  different  functions  to  play  in  the  vital  activities  in 
the  cell,  the  macronucleus  being  the  chief  seat  of  metabolic 
activities,  while  the  micronucleus  is  mainly  concerned  with 
reproduction  and  maintenance  of  the  race. 


62  ORGANISMS  OF  ONE  CELL 

Contractile  Vacuoles. — The  contractile  vacuole  of  Amoeba 
proteus  is  a  single  spherical  vesicle  which  moves  in  the  endo- 
plasm  with  the  other  endoplasmic  organs.  Paramecium  is  more 
highly  differentiated  in  this  respect  by  having  two  vacuoles 
which  are  fixed  in  the  cell,  and  open  to  the  outside  by  permanent 
pores  in  the  membrane.  One  of  the  vacuoles  is  in  the  anterior 
third  of  the  body;  the  other  in  the  posterior  third.  A  conspicu- 
ous feature  in  regard  to  them  is  that  special  canals  feed  them  by 
bringing  waste  matters  from  all  parts.  When  these  canals  are 
full  a  characteristic  radiate  structure  about  the  vacuole  can  be 
made  out  with  ease.  Systole  or  rupture  of  the  vacuole  and 
diastole  or  filling  are  independent  in  the  two  organs,  but  both 
radiate  structures  may  be  seen  at  the  same  moment  (Fig.  26). 

The  waste  matters  that  are  collected  and  excreted  through  the 
vacuoles  consist,  probably,  of  urea  and  carbon  dioxide  resulting 
from  the  processes  of  destructive  metabolism.  As  in  Amoeba, 
murexid  crystals  have  also  been  demonstrated  in  these  vacuoles 
showing  the  presence  of  uric  acid. 

Ectoplasm. — The  ectoplasm  of  Paramecium  is  much  more 
complicated  than  the  endoplasm,  and  more  so  than  the  ecto- 
plasm of  amoeba.  It  is  covered  by  a  lifeless  "pellicle/7  equiva- 
lent to  the  cuticle  of  higher  animals.  The  membrane  or  cortical 
plasm  is  relatively  thick  and  forms  a  firm  but  plastic  covering 
for  the  cell,  by  means  of  which  the  organism  retains  a  definite 
form  or  "morph."  The  cilia  are  inserted  in  it,  each  cilium 
taking  its  origin  from  a  minute  basal  granule,  from  the  substance 
of  which  it  is  apparently  formed.  The  ectoplasm  is  further 
complicated  by  the  presence  of  peculiar  rod-like  elements 
termed  trichocysts.  When  the  organism  is  irritated  in  any  way 
the  material  forming  these  trichocysts  is  shot  out  with  consider- 
able force  and  a  network  of  threads  is  formed  about  the  cell. 
The  trichocysts  thus  serve  as  a  means  of  protection  against 
small  enemies  which  are  prevented  by  the  weft  of  threads  from 
reaching  the  cell.  In  some  forms  of  Infusoria  similar  trichocysts 
have  an  offensive  as  well  as  a  protective  function,  the  minute 
organisms  being  able  to  sting  and  paralyze  other  organisms  pre- 
paratory to  devouring  them.  '  This  paralysis  is  due  to  a  minute 


PARAMECIUM  CAUDATUM,  63 

quantity  of  poison  contained  in  the  thread;  in  Paramecium, 
however,  there  is  no  evidence  to  show  that  the  trichocysts  are 
poisoned.  The  extrusion  of  trichocysts  may  be  seen  by  adding 
a  small  quantity  of  dilute  acetic  acid  to  the  medium. 

Nutrition. — Paramecium  lives  primarily  on  bacteria  which 
are  always  present  in  infusions.  A  constant  stream  of  water 
passes  into  the  mouth  and  down  the  gullet,  and  bacteria  carried 
by  the  stream  are  constantly  taken  into  the  endoplasm.  A 
gastric  vacuole  forms  at  the  bottom  of  the  gullet  which  gradu- 
ally fills  with  water  and  bacteria.  The  vacuole  is  then  carried 
away  from  the  mouth  by  the  streaming  protoplasm  (cyclosis) 
and  the  process  of  digestion  begins,  as  in  Amoeba,  by  the  secre- 
tion into  it  of  a  mineral  acid.  The  bacteria  are  killed  by  this 
acid  and  begin  to  swell  preparatory  to  dissolution.  After  from 
10  to  15  minutes  the  vacuoles  show  an  alkaline  reaction,  and 
the  further  processes  of  digestion,  requiring  several  hours,  are 
completed  in  this  alkaline  medium.  In  well-fed  Paramecia  the 
cell  becomes  loaded  with  these  vacuoles  containing  bacteria  in 
various  stages  of  digestion.  As  in  Amoeba  again,  this  later  diges- 
tion is  brought  about  by  a  proteolytic  enzyme  which  acts  like 
trypsin.  Finally  the  liquid  of  the  vacuole  disappears,  as  the 
digested  food  becomes  intimately  mixed  with  the  protoplasm, 
and  assimilation,  presumably  as  in  Amoeba  proteus,  takes  place. 

An  instructive  picture  of  the  protoplasm  of  Paramecium  can 
be  obtained  by  systematically  over-feeding  it  for  a  long 
period,  e.g.,  for  some  months  on  a  rich  hay  infusion  diet.  The 
protoplasm  becomes  filled  with  dark  granules,  the  vacuoles  of 
the  protoplasm  become  indistinct  or  lost,  the  contractile  vacu- 
oles lose  their  rhythmic  action,  and  movements  are  slow  and 
irregular.  At  such  a  time  the  organism  is  said  to  be  in  a  state 
of  depression.  It  is  loaded  down  with  reserves  of  food  partly 
digested,  and  seems  to  be  unable  to  assimilate  (Fig.  27).  If  a 
small  quantity  of  salt  be  added  (potassium  phosphate  or  potas- 
sium chloride) ,  the  dense  structure  slowly  disappears,  first  in  the 
region  around  the  nucleus;  in  a  few  days  the  protoplasm 
becomes  as  clear  and  vigorous  as  ever.  Such  experiments  show 
either  that  some  of  the  oxidative  ferments  are  exhausted  so  that 


64 


ORGANISMS  OF  ONE  CELL 


the  organisms  become  gradually  overpowered  by  the  products 
of  their  own  metabolism,  or  that  the  formative  processes  are 
not  properly  functioning.  They  also  show  that  salts  or  elec- 
trolytes were  probably  no  longer  present  in  the  cells  and  that 
oxidative  processes  had  ceased,  because  the  simple  addition  of 
salts  to  the  medium  resulted  in  the  restoration  of  vital  activities. 
Finally  they  indicate  that  the  nucleus  of  the  cell  is  probably  the 
seat  of  manufacture  of  the  oxidative  ferments"  or  catalyzers. 


FIG.  27. — Paramecium  caudatum  in  the  condition  of  depression  and  recovery 
through  the  use  of  salts.  The  individual  on  the  left  has  densely  packed  proto- 
plasm, the  others  were  similar  individuals  which  were  treated  with  potassium 
phosphate.  From  photographs  of  prepared  specimens. 

Reproduction. — Like  Amoeba  proteus  and  the  flagellates, 
Paramecium  reproduces  by  simple  division,  the  micronucleus 
dividing  first.  The  cell  divides  transversely  through  the 
middle  of  the  macronucleus  which  is  passively  divided  with  the 
rest  of  the  cell  (Fig.  28).  One  new  mouth  and  new  contractile 
vacuoles  are  formed  by  the  daughter  cells  which  separate  and 
begin  an  independent  existence.  The  entire  process  requires 
from  half  an  hour  to  two  hours  according  to  the  temperature. 

Irritability. — Like  A  moeba  proteus  again,  Paramecium  responds 
to  external  stimuli,  but  having  definite  motile  organs  its  re- 
sponses are  more  definite  and  more  easily  studied.  It  answers 


PARAMECIUM  CAUDATUM 


65 


to  all  sudden  stimuli  by  a  definite  reaction  termed  a  "  mo  tor 
response."  It  backs  away  by  reversal  of  its  ciliary  action,  then 
turns  on  its  axis  and  moves  forward  again.  If  the  offending 
object  is  still  encountered  it  repeats  the  action  until  finally  its 
forward  movement  is  unimpeded.  It  reacts  definitely  to  the 
action  of  a  galvanic  current,  and  always  moves  toward  the  nega- 
tive pole,  thus  showing  a  well-marked  galvano taxis.  With 
strong  acids  and  alkalis  it  gives  the  characteristic  motor 
response. 


EXPLODED  TRICHOCYSTS_  ± 


TRICHOCYSTS 


DIV/DfO  MACffONUCLEUS. 


DIVIDING  WCRONUCL£US. 


DIVIDED  MftCRONUCLEUS 


FIG.  28. — Paramecium  in  division.     Photograph  of  a  section. 

D.  SOME  GENERAL  BIOLOGICAL  PROBLEMS  ASSOCIATED  WITH 

PROTOZOA 

The  single-celled  animals  have  been  intimately  connected 
with  some  of  the  deepest  problems  in  general  biology,  for  since 
vital  manifestations  are  everywhere  the  same,  physiologists 
have  turned  for  their  solution  to  these  simple  organisms  where 
processes,  like  structures,  are  relatively  uncomplicated.  Some 
of  these  problems,  such  as  the  distinction  between  animals  and 
plants,  have  only  an  academic  importance;  another,  spontane- 
ous generation,  has  only  an  historical  importance,  but  others, 


66  ORGANISMS  OF  ONE  CELL 

like  old  age,  the  origin  of  death,  the  significance  of  fertilization 
are  all  deep  problems  which  rest  upon  the  very  foundations  of 
biological  science. 

Animals  and  Plants. — At  first  glance  it  would  appear  to  be  a 
simple  matter  to  distinguish  between  animals  and  plants,  and 
superficial  observers  do  not  hesitate  to  do  so  with  full  assurance. 
But  when  we  come  to  examine  them  more  closely  we  find  that 
no  definite  boundary  line  can  be  drawn  between  them,  and  no 
single  characteristic  belongs  absolutely  to  one  or  the  other 
group.  Formerly  it  was  argued  that  plants  are  quiescent  while 
animals  move,  and  the  power  of  spontaneous  movement  was 
regarded  as  sufficiently  characteristic  to  distinguish  them.  But 
numerous  sensitive  plants,  the  Venus  fly  trap  for  example,  and 
many  algae  have  the  power  of  movement  quite  as  well  developed 
as  do  many  animals,  while  many  animals,  as  for  example  the 
sponges,  are  quiescent.  Again  it  was  thought  that  chlorophyll, 
giving  well-defined  colors  to  plants,  is  a  definite  and  distinc- 
tive feature.  But  many  animals  such  as  Euglena  and  many 
other  flagellates  are  similarly  colored  by  chlorophyll.  The 
presence  of  chlorophyll  indicates  a  power  of  manufacturing 
starch  and  sugars,  while  plant  cells  generally  have  a  definite 
membrane  of  cellulose  which  is  closely  allied  to  starch  in  chem- 
ical composition.  Cellulose  therefore  was  also  regarded  as  a 
specific  plant  characteristic.  But  again  a  number  of  animals 
have  the  power  of  manufacturing  cellulose;  the  Dinoflagellates, 
for  example,  have  a  thick  cellulose  wall,  while  some  of  the  higher 
animals,  notably  the  group  known  as  the  ascidians,  have  well- 
defined  tests  of  the  same  material.  Still  later  it  was  maintained 
that  plants  do  not  eat  solid  food  in  the  form  of  proteins  while 
animals  do;  but  the  Venus  fly  trap  Dionaea  not  only  moves 
but  also  catches  and  digests  insects  of  various  kinds.  On  the 
other  hand,  a  number  of  animals,  especially  the  unicellular  ones, 
do  not  take  in  solid  food  but  manufacture  it,  as  do  the  plants, 
through  the  aid  of  chlorophyll.  Similarly  with  every  other 
distinctive  feature  we  find  some  exceptions  on  one  side  or  the 
other,  showing  that  physiological  processes  in  nature  are  no- 
where monopolized  by  any  one  type  of  organisms. 


SPONTANEOUS  GENERATION  67 

While  it  is  impossible  to  draw  a  definite  line  between  animals 
and  plants  it  is  possible,  nevertheless,  through  the  sum  of  char- 
acters to  determine  whether  an  organism  is  either  plant  or 
animal,  or  some"  form  of  life  intermediate  between  them.  For 
the  determination  of  a  given  questionable  type  it  is  necessary 
to  take  into  consideration  not  only  form,  movement,  and  mode 
of  nutrition  but  also  the  immediate  relations.  Thus  Euglena 
has  many  of  the  physiological  characteristics  of  plants  of  which 
the  mode  of  nutrition  is  the  most  important;  but  it  itself  and  a 
nearly-related  type,  Chromulina  flavicans,  have  the  power  of 
both  holophytic  and  holozoic  nutrition  and  can  live  in  the  dark 
on  solid  protein  matter,  or  in  the  light  without  solid  food  where 
it  manufactures  its  food.  An  allied  form,  Astasia,  lives  solely 
on  solids.  The  structures  and  life  history  of  Euglena  place  it 
unmistakably  with  the  animal  flagellates. 

It  is  now  known  that  plants,  like  animals,  renew  their  proto- 
plasm with  oxygen,  salts  and  proteins,  and  give  off  CO2  and 
other  waste  matters  the  same  as  animals  do,  the  only  essential 
difference  being  their  power  to  manufacture  the  proteins  to  be 
used  as  food.  Their  functions,  therefore,  are  fundamentally 
constructive  while  animals  are  destructive;  all  plant  tissues 
and  organs  are  differentiated  to  subserve  this  great  function 
while  those  of  animals  are  mainly  differentiated  for  the  pro- 
curing of  food,  digesting  and  assimilating  it.  The  two  great 
lines  of  living  things  have  thus  developed  in  different  directions, 
and  the  higher  we  go  in  either  scale  the  more  easily  we  are  able 
to  distinguish  between  anknals  and  plants  by  these  structural 
differences. 

Spontaneous  Generation. — "But  expectation  is  permissible 
where  belief  is  not;  and  if  it  were  given  me  to  look  beyond  the 
abyss  of  geologically  recorded  time  to  the  still  more  remote 
period  when  the  earth  was  passing  through  physical  and  chem- 
ical conditions  which  it  can  no  more  see  again  than  a  man  can  re- 
call his  infancy,  I  should  expect  to  be  a  witness  of  the  evolution 
of  living  protoplasm  from  non-living  matter."  (Huxley,  Bio- 
genesis and  Abiogenesis.) 

All  biologists  are  practically  agreed  that  living  matter  origi- 


68  ORGANISMS  OF  ONE  CELL 

nated  on  the  earth's  surface  from  salts  and  other  inorganic 
matter  at  a  time  when  conditions  of  temperature,  atmosphere 
and  other  physical  characteristics  of  the  globe  were  very  differ- 
ent from  the  conditions  today.  At  the  present  time,  while 
ignorant  of  the  first  causes  all  are  agreed  that  living  matter 
cannot  arise  spontaneously  from  non-living  matter,  and  that  all 
plants  and  all  animals  come  from  the  germs  of  their  ancestors. 
All  theories  to  the  contrary  have  been  based  upon  ignorance, 
and  the  gradual  clearing  away  of  these  dark  clouds  forms  an 
interesting  chapter  in  modern  biology.  A  characteristic  of  the 
human  mind  is  to  explain  what  cannot  be  seen  or  comprehended, 
by  the  most  plausible  hypotheses  based  upon  what  is  known. 
This  is  why  in  the  lyth  century  it  was  generally  believed  that 
insects  are  spontaneously  generated  in  decaying  meat,  the  myth 
being  disproved  by  the  simple  experiment  of  Redi,  an  Italian 
naturalist,  who  kept  fresh  meat  under  a  fine  netting  upon  which 
he  saw  flies  deposit  their  eggs;  these  he  watched  develop  into 
maggots  and  later  into  flies.  Thus  a  set  of  phenomena  was 
removed  from  the  unknown  into  the  known,  and  Redi  concluded 
from  his  observation  that  all  living  things  come  from  pre- 
existing living  things,  a  conclusion  formulated  in  the  well-known 
dictum  credited  to  Harvey  (?),  omne  vivum  ex  vivo.  RedPs  con- 
clusions, however,  were  not  to  be  fully  accepted  for  more  than 
two  hundred  years,  for  shortly  after  his  experiments  were  carried 
out,  the  world  of  microscopic  organisms  was  discovered  by  the 
Dutch  naturalist  Leeuwenhoek,  and  ignorance  of  their  origin 
resulted  in  a  new  life  for  the  theory  of  spontaneous  generation. 
This  theory,  therefore,  finally  died  out  only  in  our  own  times 
with  the  famous  experiments  of  Pasteur  and  Tyndall  upon  the 
smallest  visible  forms  of  living  organisms,  bacteria  and  yeasts. 
The  Problem  of  Age  and  Natural  Death. — With  full  and  un- 
hindered processes  of  metabolism  there  is  no  a  priori  reason  why 
living  matter  contained  in  the  single-celled  organism,  apart  from 
tragic  or  accidental  death,  should  not  live  indefinitely.  So 
far  as  we  know,  however,  all  living  things  experience  a  weakening 
of  these  fundamental  biological  activities,  and  pass  through  a 
longer  or  shorter  period  of  physiological  weakness,  which  we 


AGE  AND  NATURAL  DEATH 


69 


term  old  age,  ending  in  natural  death.  The  length  of  life  varies 
within  wide  limits,  animals  on  the  whole  having  shorter  lives 
than  plants;  some  of  the  giant  trees  of  California,  for  example, 
live  for  tens  of  centuries  while  some  of  the  insects  (May-flies) 
are  born  and  live  out  their  adult  life  within  a  single  day.  On  the 
other  hand,  some  animals  like  the  tortoise  may  live  for  hundreds 
of  years. 

Senescence. — In  the  higher  animals  or  metazoa  the  cells  are 
differentiated  for  the  performance  of  different  functions,  and  the 
various  activities  of  metabolism  are 
relegated  to  different  types  of  special- 
ized cells.  These  are  the  links  in 
the  chain  of  vital  phenomena  which 
weaken,  give  out  and  lead  to  old  age. 
The  secreting  cells,  for  example,  ulti- 
mately cease  functioning  one  by  one, 
and  their  places  in  the  tissue  are 
taken  by  non-functioning  connective- 
tissue  cells ;  when  enough  of  these  are 
thus  worn  out  and  replaced,  activity 
of  the  organ  is  impaired  and  the 
general  vitality  of  the  entire  organism 
is  correspondingly  weakened.  In  the 
human  organism  this  process  results 
in  hardening  of  the  tissues,  leading, 
for  example,  to  cirrhosis  of  the  liver 
or  kidney,  sclerosis  of  arteries,  etc., 
ending  inevitably  in  death  after  a 
longer  or  shorter  time. 

The  problem  of  old  age  therefore 

resolves  itself  into  the  question,  why  do  the  individual  cells 
give  out?  It  would  seem  that  these  differentiated  cells  of  the 
body  are  endowed  with  a  limited  possibility  of  action,  or  with 
a  " potential  of  vitality,"  which  is  gradually  exhausted  by 
continued  use.  Yet  some  of  these  epithelial  cells,  under  cir- 
cumstances abnormal  to  the  organism,  have  the  capacity  to  live 
far  beyond  the  limits  of  the  natural  life  of  the  organism  to 


W- 


FIG.  29. — Paramecinm 
caudatum  in  conjugation. 
The  micronuclei  can  be 
seen  in  the  process  of  di- 
viding. The  organisms  are 
united  in  the  peristome 
region.  From  a  photograph 
of  a  preparation. 


70  ORGANISMS  OF  ONE  CELL 

which  they  belong.  Thus  epithelial  cells  of  the  mouse,  under 
abnormal  conditions  which  we  call  cancer,  may  be  transplanted 
from  mouse  to  mouse  for  years  after  the  normal  length  of 
life  of  the  original  animal,  a  fact  demonstrating  that  epi- 
thelial cells,  at  least  under  these  abnormal  conditions,  have 
a  far  greater  potential  of  vitality  than  is  represented  by 
the  ordinary  length  of  life  of  the  individual.  Weakness  and 
death  of  the  individual  cells,  therefore,  must  be  due  to  some  de- 
fect in  the  inter-relations  of  the  many  differentiated  cells  and 
organs  of  the  body  of  the  metazoon;  this  defect  is  cumulative 
until  the  organisms  are  unable  to  carry  on  the  necessary 
functions,  and  die.  The  same  result  may  be  brought  about  by 
the  local  destruction  of  cells  through  disease;  thus  bacteria 
may  destroy  the  cells  of  the  lungs  in  pulmonary  tuberculosis, 
a  loss  resulting  in  the  weakening  of  other  cells  in  the  body,  and 
finally  in  death  through  inability  to  obtain  oxygen  and  gives  off 
C02. 

This  gradual  weakening  of  the  cells  and  vital  activities  in 
general  led  Weismann,  a  famous  German  biologist,  to  the  con- 
clusion that  old  age  and  natural  death  are  penalties  which  the 
higher  organisms  must  pay  for  their  privilege  of  differentiation, 
while  the  unicellular  organisms,  dependent  only  upon  themselves, 
have  an  unlimited  capacity  of  life,  i.e.,  are  potentially  immortal. 
These  conclusions  have  been  repeatedly  challenged  on  the 
ground  that  all  protoplasm  is  subject  to  the  same  laws  of 
physiological  usury,  and  numerous  experiments  beginning 
with  those  of  Maupas,  1888,  have  been  undertaken  to  show  that 
protozoa,  like  metazoa,  undergo  senescence,  arid  die  from  old 
age.  It  follows  from  the  negation  of  spontaneous  generation, 
however,  that  all  living  things  are  composed  of  protoplasm  that 
has  been  living  continuously  since  life  appeared  on  the  earth,  and 
it  is  obvious  that  all  organisms  contain  some  cells  endowed  with 
the  potential  of  physical  immortality.  These,  called  the  germ 
cells,  hand  down  the  race  from  generation  to  generation.  The 
experiments  of  Maupas  and  of  subsequent  investigators  have 
shown  that  it  is  only  in  this  sense  that  protozoa  are  physically  im- 
mortal. Maupas  isolated  Oxytricha,  Stylonychia,  and  other 


REJUVENESCENCE  71 

unicellular  animals,  and  kept  the  descendants  isolated,  in  order  to 
prevent  fertilization,  through3i6,  319,  etc.,  generations  of  simple 
division.  Toward  the  end  of  the  time  the  cells  became  reduced 
in  size  and  abnormal  in  structure  through  loss  of  cilia  and  other 
organs,  and  all  finally  died  under  conditions  similar  to  senile 
degeneration  in  higher  animals.  Many  other  types  of  Infusoria, 
also,  have  been  watched  through  many  hundreds  of  generations 
by  different  observers,  but  the  physiological  activities  in  every 
such  experiment  save  one,  weakened,  and  sooner  or  later, 
abnormal  or  deformed  specimens  brought  the  race  to  an  end. 
The  one  apparent  exception  is  the  case  of  Paramecium  aurelia 
which  Woodruff  has  carried  on  for  many  years  (over  6000 
generations)  without  conjugations.  At  regular  intervals, 
however,  the  race  of  Paramecium  under  observation  showed  a 
reduced  vitality  as  measured  by  the  division  rate,  and  it  was 
found  that,  during  these  periods  of  depression,  the  macronu- 
cleus  breaks  up  into  fragments,  and  the  micronuclei  divide. 
New  macronuclei  develop  from  some  of  the  products  of  division 
of  the  micronuclei,  functional  micronuclei  from  others,  while 
some  degenerate.  The  fragments  of  the  old  macronucleus 
and  the  degenerating  micronuclei  are  absorbed  in  the  cytoplasm, 
and  this  addition  of  relatively  large  quantities  of  nucleo-proteins 
makes  a  decided  change  in  the  chemical  organization  of  the  en- 
tire protoplasm.  As  a  result  of  this  process  of  nuclear  and  cyto- 
plasmic  reorganization  which  Woodruff  calls  endomixis,  all  of 
the  vital  activities  are  accelerated  so  that  the  organisms  grow 
and  divide  at  a  more  rapid  r,ate  than  during  the  process  of 
endomixis  or  just  prior  to  it.  This  high  vitality  then  gradu- 
ally decreases  until  the  next  period  of  depression  when  the 
process  of  endomixis  is  repeated. 

In  essence,  therefore,  it  appears  that  the  unicellular  organisms 
agree  with  the  multicellular  in  possessing  physiological  powers 
which  gradually  wear  out.  To  be  sure,  the  single  cells  of  the 
majority  of  generations  do  not  die.  They  cease  to  live  as  the 
same  cells,  but  the  protoplasm  continues  to  live  in  the  daughter 
cells  arising  from  divisions ;  but  the  same  is  true  of  any  individ- 
ual cell  of  a  metazoon,  since  the  adult  organism  is  formed  by 


72  ORGANISMS  OF  ONE  CELL 

the  continued  division  of  a  single  original  egg  cell.  The 
entire  race  of  Paramecium  derived  from  a  single  ancestral  cell, 
and  not  any  single  cell  of  that  race  should  therefore  be  compared 
with  a  metazoon,  the  race  of  Paramecium  in  most  cases  may 
die  from  old  age  no  less  surely  than  the  race  of  cells  composing 
the  metazoon.  Old  age  and  natural  death,  therefore,  appear 
to  be  'characteristic  of  animals  whether  single  cells  or  many 
celled. 

In  spite  of  the  fact  that  protozoa,  should  they  escape  the  thou- 
sands of  their  natural  enemies,  may  die  of  old  age,  they  never- 
theless exist  in  more  or  less  abundance  in  natural  waters,  and 
will  undoubtedly  continue  to  exist  in  the  future.  The  question 
then  arises,  how  is  this  physiological  weakness  overcome  and 
what  means  are  employed  in  nature  to  perpetuate  the  species? 
One  means  of  accomplishing  this  end  is  the  chemical  reorgani- 
zation following  endomixis  as  described  above;  another  and 
an  essentially  similar  means  is  the  chemical  reorganization  of 
the  protoplasm  which  follows  fertilization  through  the  act  of 
conjugation.  Biitschli  in  1876  observed  that  a  culture  of  Para- 
mecium, after  some  weeks  in  a  watch  glass,  showed  hundreds 
of  pairs  in  conjugation,  the  cells  being  united  in  the  region  of 
the  peristomial  area.  This  union  or  conjugation,  lasting  from 
eighteen  to  twenty-four  hours,  is  followed  by  separation  of  the 
two  individuals  (Fig.  29).  From  this  observation  Biitschli 
concluded  that  conjugation  is  for  the  purpose  of  renewing  the 
vitality  of  the  race,  or  is  a  means  of  protoplasmic  rejuvenes- 
cence, and  this  interpretation,  while  it  has  been  questioned, 
has  never  been  disproved  or  improved. 

Conjugation  or  Fertilization. — Conjugation  of  protozoa  is 
essentially  the  same  as  fertilization  in  metazoa,  and  in  one  form 
or  another  represents  a  phenomenon  practically  universal  in 
animals  and  plants.  It  is,  therefore,  one  of  the  fundamental 
activities  of  living  things  (see  page  15).  It  is  usually  associated 
with  reproduction,  but  reproduction  may  go  on  without  it,  as 
in  the  case  of  division,  spore  formation,  etc.,  of  the  protozoa, 
or  budding  in  Hydra  and  plants,  or  parthenogenesis  in  insects. 
Strictly  speaking,  therefore,  it  is  not  a  process  of  reproduc- 


EFFECTS  OF  CONJUGATION 


73 


tion  but  a  process  of  protoplasmic  reorganization,  followed 
by  renewal  or  re-birth  of  all  vital  activities  including  that  of 
reproduction. 


FIRST    MATURATION    DIVISION  OF  MICRONUCLEUS 


SECOND  AND    THIRD 
DIVISION  OF  MICRONUCLEUS 


THREE   SOMATIC  DIVISIONS   OF  FERTILIZED   NUCLEU3 


FERTILIZATION 


TWO  CONSECUTIVE  DIVISIONS 
GIVING   FOUR  NORMAL   CELLS 


FIG.  30. — Diagram  of  the  consecutive  stages  in  conjugation  of  Paramecium 

caudatum. 

Investigations  subsequent  to  those  of  Butschli  have  revealed 

all  the  happenings  during  the  process  of  conjugation  in  Para- 

'mecium.     The  micronuclei  in  each  cell  first  begin  to  swell  by 

the  absorption  of  fluids  from  the  endoplasm;  the  chromatin 


74  ORGANISMS  OF  ONE  CELL 

increases  enormously  in  quantity  and  becomes  drawn  out  in 
the  form  of  rods  termed  chromosomes,  too  numerous  to  count. 
Each  of  these  chromosomes  is  then  divided  into  two  equal  parts, 
after  which  the  micronuclei  divide  through  the  center,  each 
daughter  micronucleus  receiving  one-half  the  original  chroma- 
tin.  There  are  now  two  micronuclei  in  each  cell,  and  all  four 
divide  once  again,  forming  eight  in  all  or  four  in  each  cell.  Of 
these  four,  three  degenerate  and  are  absorbed  in  the  endoplasm, 
leaving  one  micronucleus  in  each  cell.  These  then  divide  once 
again,  forming  what  are  called  pronudei,  one  of  which  migrates 
from  its  cell  into  the  other  cell  so  that  a  mutual  exchange  of 
pronuclei  takes  place.  Each  wandering  pronucleus  unites 
with  the  stationary  pronucleus  of  the  opposite  cell  and  fuses 
completely  with  it,  forming  a  fertilization  nucleus  (Fig.  30, 
A-H). 

In  the  meantime  the  macronucleus  of  each  cell  begins  to 
break  into  pieces  and  to  degenerate,  and  sooner  or  later  it  en- 
tirely disappears,  although  this  final  disappearance  does  not 
occur  until  some  time  after  the  two  cells  separate  and  divisions 
have  begun.  After  separation  of  the  conjugating  cells  the 
fertilization  nucleus  gives  rise,  by  divisions,  to  eight  micro- 
nuclei.  Four  of  these  begin  to  swell,  change  in  character, 
and  develop  into  four  new  macronuclei.  After  twenty- 
four  to  forty-eight  hours  the  exconjugant  divides.  Two 
macronuclei  and  two  micronuclei  pass  into  each  daughter 
cell,  and  after  another  twenty-four  hours  these  cells  divide 
again,  one  micronucleus  and  one  macronucleus  going  to  each 
daughter  cell.  With  this  final  division  the  normal  relations 
of  the  cell  are  restored,  and  the  processes  of  conjugation 
are  ended,  the  resulting  cells  having  each  one  macronucleus  and 
one  micronucleus  (Fig.  30,  I-P). 

With  the  breaking  up  of  the  old  macronucleus  the  protoplasm 
becomes  loaded  with  nuclear  stuffs,  as  in  endomixis,  and  these 
probably  renew  the  supply  of  material  for  the  production  of 
the  various  endoenzymes  needed  in  the  vital  reactions. 

With  the  conjugation  of  Paramecium,  therefore,  the  ordinary 
cells  are  metamorphosed  into  germ  cells  with  extraordinary 


EFFECTS  OF  CONJUGATION  75 

activities,  the  protoplasm  of  the  race  is  completely  reorganized 
by  the  interchange  of  nuclear  material,  by  the  formation  of  new 
macronuclei  and  new  micronuclei  of  a  different  chemical 
make-up,  and  by  the  formation  of  a  new  cytoplasm  through  the 
absorption  of  the  old  macronucleus  and  three-quarters  of  the 
old  micronucleus.  It  now  contains  the  potential  of  a  new 
race  of  cells,  exactly  as  in  the  case  of  a  fertilized  metazoan  egg, 
and  in  the  race  of  cells  derived  from  it,  some  will  be  germ  cells, 
just  as  in  the  case  of  the  metazoan.  So  far  as  immortality 
is  concerned,  therefore,  the  unicellular  organisms  resemble 
the  multicellular;  there  is  a  reorganization  of  the  cytoplasm, 
a  new  combination  of  physico-chemical  elements,  and  a  new 
individuality  in  protozoa,  exactly  as  in  metazoa.  There  is  no 
essential  difference  in  the  vital  phenomena,  only  a  difference 
in  their  manifestations.  Old  age,  followed  by  natural  death,  is  a 
biological  and  inevitable  termination  of  vital  activities,  which,  so 
far  as  we  know,  can  only  be  prevented  by  fertilization,  or 
by  endomixis  the  equivalent  of  parthenogenesis,  and  then 
only  in  the  germ  cells,  since  epithelial  cells,  muscle  or  nerve 
cells  have  no  possibility  of  similar  reorganization. 


CHAPTER  IV 
ORGANISMS  OF  TISSUES 

VERY  little  observation  is  needed  to  show  that  a  Paramecium 
is  a  complicated  mechanism,  despite  the  fact  that  it  is  only  a 
single  cell.  The  various  parts  of  the  cell  are  differentiated  for 
the  performance  of  different  functions,  whereas  in  Metazoa  the 
same  functions  are  performed  by  aggregates  of  cells  and  tissues. 
Such  cells  and  tissues,  like  the  parts  of  the  protozoan  cell,  are 
specialized  for  different  functions.  Fundamentally  alike  in 
their  physiological  activities,  there  is,  nevertheless,  a  vast  dif- 
ference between  organisms  of  one  cell  and  organisms  of  many 
cells.  With  the  aggregate  of  cells  there  is  a  great  possibility  of 
differentiation  which  is  absent  in  unicellular  forms,  and  with  this 
differentiation  the  possibility  of  structural  complications  is 
vastly  increased. 

Could  we  collect  and  hold  together  all  the  progeny  of  a  fertil- 
ized Paramecium  cell  into  a  harmoniously  working  whole,  the 
result  would  be  an  organism  composed  of  tissues,  or  in  this  case, 
of  a  single  tissue,  for  all  the  cells  would  be  fundamentally  alike 
and  performing  the  same  functions.  While  this  condition  does 
not  exist  for  Paramecium,  there  are  protozoa  in  which  the 
progeny  after  division  remain  connected,  forming  aggregates 
of  many  cells.  These  protozoan  aggregates,  colonies,  are  des- 
ignated according  to  their  method  of  formation,  as  gregaloid, 
sphaeroid,  arboroid  and  catenoid.  In  all  of  these,  except  the 
first,  the  colony  results  from  the  incomplete  division  of  the 
cells  and  their  products.  Gregaloid  colonies  differ  in  that 
they  are  formed  by  the  coalescence  of  adult  cells  into  a  loose 
group,  as  in  the  case  of  Microgromia  socialis.  A  catenoid  colony 
is  formed  by  the  attachment,  end  to  end  or  side  by  side,  of  sister 
cells  resulting  from  division.  An  arboroid  colony  is  formed  by 
the  daughter  cells  of  one  organism  remaining  attached  to  the 

76 


CELL  AGGREGATES 


77 


parent;  both  parent  and  offspring  then  reproduce  in  the  same 
way  until  a  branching  tree-like  colony  results  (Fig.  31). 
Sphaeroidal  colonies,  finally,  differ  from  the  others  in  that  the 
cells  resulting  from  division  remain  embedded  in  a  gelatinous 
matrix  secreted  by  the  parent  organism  (Fig.  24).  Such 
colonies  are  usually  spherical,  the  constituent  cells  being  ar- 
ranged about  the  periphery.  When  this  condition  in  develop- 


/ 


FIG.  31. — An  arboroid  colony  of  flagellated  protozoa,  Codosiga  cymosa,  Sav. 
Kent.     (From   Calkins   after   Kent.) 

ment  is  reached  it  is  not  a  long  step  to  the  simplest  type  of 
metazoan,  and  we  find  cases  where  there  is  even  a  regional  dif- 
ferentiation in  the  colony.  A  Proterospongia,  for  example,  is 
a  gelatinous  mass  with  cells  arranged  about  the  periphery; 
when  abundantly  fed  these  cells  migrate  one  by  one  to  the  in- 
terior of  the  jelly  mass,  losing  their  flagella  in  the  process. 
Within  the  jelly  they  divide,  and  the  cells  then  wander  back  to 
the  periphery  to  take  their  place  with  the  feeding  cells  of  the 
colony.  Here  then  is  a  temporary  differentiation  into  vegeta- 


78 


ORGANISMS  OF  TISSUES 


live  or  somatic  cells,  and  reproducing  or  reproductive  cells. 
The  differentiation  is  carried  a  step  farther  in  the  case  of  Pleo- 
dorina  where  twenty-eight  of  the  cells  are  capable  of  reproduc- 
ing, while  the  remaining  four  cells,  making  up  the  thirty-two 
cell  colony,  are  purely  vegetative  and  do  not  reproduce.  Here 
there  is  a  permanent  differentiation  in  the  colony  and  a  long 
step  toward  the  metazoan  condition.  In  some  colonies  finally, 
as  in  Gonium  perforate,  the  method  of  development  approaches 


FIG.  32. — Reproduction  of  Gonium  pectorale.  Each  of  the  sixteen  cells  of  the 
ordinary  colony  (see  Fig.  25)  divides  until  a  sixteen-cell  stage  results;  the  old 
colony  then  breaks  up  and  the  sixteen  young  colonies  grow  independently. 

closely  to  that  of  metazoa.  The  organism  consists  of  sixteen 
cells  arranged  as  in  Fig.  25.  When  ready  to  reproduce,  each  cell 
of  the  colony  divides  first  into  two  cells;  these  cells  do  not  sepa- 
rate but  while  still  connected  they  divide  again  into  four,  these 
four  divide  into  eight,  and  the  eight  ultimately  into  sixteen. 
Each  cell  of  the  parent  organism,  therefore,  gives  rise  to  a  new 
colony  of  sixteen  cells,  each  of  which  is  liberated  as  a  colony 
by  dissolution  of  the  original  jelly  mantle  (Fig.  32).  Here  then 
is  a  process  of  development,  an  embryology  in  the  case  of  a  pro- 
tozoon,  in  which  all  of  the  constituent  cells  are  potentially 


CLEAVAGE  IN  METAZOA  79 

germ  cells.  Furthermore,  the  first  two  division  planes  are  ver- 
tical and  the  third  horizontal,  exactly  as  in  the  case  of  holoblas- 
tic  cleavage  in  metazoa. 

The  primordial  cell  (fertilized  egg  cell)  of  a  higher  animal 
develops  by  continual  cell  division  until  myriads  of  cells  con- 
stituting the  adult  organism  are  formed.  In  the  typical  case 
and  in  the  majority  of  forms  the  early  stages  of  this  development 
follow  the  course  illustrated  in  Fig.  33 .  The  process  of  holoblas- 
tic  cleavage,  so-called,  is  a  regular  and  symmetrical  division  of 
the  egg  cell.  The  first  cleavage  in  a  vertical  plane  results  in 
the  formation  of  two  similar  cells — the  two-cell  stage;  the  second 
cleavage,  also  vertical,  gives  four  similar  cells;  the  third  cleav- 
age differs  in  being  horizontal,  crossing  the  first  two  planes  at 
right  angles  and  resulting  in  eight  cells;  in  the  majority  of  cases 
this  third  cleavage  brings  about  the  first  trace  of  differentiation 
in  the  cells,  those  of  one  pole  (vegetative)  being  larger  and  con- 
taining more  yolk  than  those  of  the  smaller  pole  (animal).  The 
fourth  cleavage  is  again  vertical,  and  the  difference  between  the 
two  poles  is  further  emphasized,  the  sixteen  cell  stage  present- 
ing eight  larger  vegetative  and  eight  smaller  animal  cells.  At 
this  stage  also  the  cells  begin  to  separate,  leaving  a  cavity  in 
the  center.  This  cavity,  termed  the  segmentation  cavity,  in 
the  majority  of  types  is  later  closed  up,  and  plays  no  part  in  the 
adult  organism.  After  the  sixteen  cell  stage  cleavage  becomes 
more  or  less  irregular,  the  cells  of  the  animal  pole  dividing  more 
rapidly  than  those  of  the  vegetative,  until  a  many  celled  hollow 
sphere  results.  At  this  stage  the  organism  is  termed  a  blastula 
(Fig.  33).  Up  to  this  time  the  developing  metazoon  differs  but 
little  from  some  colony  forms  of  protozoa.  After  the  blastula 
stage  a  step  in  development  is  taken  which  is  found  nowhere  but 
in  metazoa,  and  represents  therefore  a  great  advance  over 
protozoa.  The  cells  of  the  lower  pole  invaginate  or  turn 
in  until  their  inner  sides  come  in  contact  with  the  inner  sides  of 
the  cells  of  the  animal  pole.  This  process  is  termed  gastrulation. 
The  gastrula  thus  formed  is  a  double  walled  sack  enclosing  a 
cavity,  which  becomes  the  enteric  or  digestive  cavity  of  the 
embryo,  and  in  most  cases  the  digestive  tract  of  the  adult  is 


80 


ORGANISMS  OF  TISSUES 


formed  directly  from  this  enteric  cavity.  Because  of  this  ulti- 
mate fate  the  cavity  is  called  the  archenteron  or  primitive  gut, 
while  the  opening  to  the  outside  is  termed  the  llastopore  or  larval 
mouth.  At  this  stage  in  development  the  water  dwelling  types 
usually  leave  the  egg  membrane  and  swim  about  in  the  water, 
taking  in  food  through  the  blastopore  and  digesting  it  in  the 
archenteric  cavity.  We  see,  therefore,  that  the  larger  cells  of 


EARLY  CLemacE  STAGES 


FIG.  33. — Holoblastic  cleavage  and  gastrulation  of  Amphioxus.    P.B.,  polar  body 
(see  p.  213).     (From  Ziegler  models.) 

the  vegetative  pole  of  the  blastula  become  the  functioning  cells 
of  the  digestive  tract  of  the  adult  and  are  internal  in  position, 
forming  a  lining  of  cells  called  the  hypoblast  or  endoderm.  The 
cells  of  the  animal  pole  remain  outside,  covering  the  endoderm 
cells  and  forming  a  continuous  sheet  of  cells  termed  the  epiblast 
or  ectoderm.  The  two  layers  together  constitute  the  primary 
germ  layers  of  the  organism.  At  first  each  sheet  consists  of 


GERM  LAYERS  81 

entirely  similar  cells  having  a  similar  function,  but  as  develop- 
ment progresses  the  cells  become  differentiated  in  groups  for 
the  performance  of  different  functions,  some  cells  of  the  ecto- 
derm forming  the  outer  covering  or  skin,  the  nervous  system, 
etc.,  while  cells  of  the  endoderm  become  differentiated  for  dif- 
ferent processes  of  digestion.  Tissues  are  aggregates  of  similar 
cells  having  the  same  function.  In  the  gastrula  there  are  two 
tissues,  endoderm  and  ectoderm,  but  in  later  development 
many  different  tissues  are  formed  from  these  two,  e.g.,  epithelial, 
nerve,  muscle,  connective  tissue  and  the  like.  In  the  majority 
of  higher  animals  a  third  germ  layer,  termed  the  mesoderm,  is 
formed  between  the  ectoderm  and  the  endoderm.  This  third 
layer  gives  rise  to  muscle  tissues,  endothelial,  supporting  or 
connective,  and  germinal  tissues  (see  Chapter  VI).  Organs  are 
aggregates  of  tissues  for  the  performance  of  one  function;  diges- 
tive organs,  the  stomach,  liver,  etc.,  consist  of  secreting,  muscu- 
lar, nerve,  vascular,  and  connective  tissues.  Organs  finally  are 
grouped  in  systems  for  the  performance  of  the  fundamental 
vital  processes  of  metabolism.  The  digestive  system  includes  all 
of  the  organs  necessary  for  the  digestion  of  food;  the  muscular 
and  supporting  systems,  the  organs  of  locomotion;  the  excretory 
system,  the  organs  for  disposing  of  waste  matters;  the  respira- 
tory system,  the  organs  for  obtaining  oxygen  and  removing  CO2j 
the  nervous  system,  the  organs  for  receiving  and  transmitting 
stimuli,  and  the  reproductive  system,  the  organs  for  maintaining 
the  race.  At  the  bottom  of  all  of  the  complicated  structures  is 
the  single  cell,  the  minute,  active,  and  mysterious  unit  of  living 
matter.  Cells  form  the  tissues ;  tissues  form  organs ;  organs  form 
systems;  and  the  systems  working  harmoniously  together  form 
the  normal  living  organisms. 

All  types  of  metazoa  start  with  an  analogous  egg-to-gas- 
trula  stage  in  development,  and  differentiation  begins  from  this 
point,  although  in  many  cases  differentiation  may  begin  even 
before  this.  Naturalists  divide  the  animal  kingdom  into  great 
groups,  termed  phyla,  according  to  the  degree  and  nature  of  the 
differentiation  which  follows  from  this  gastrula  stage.  The 
simplest  of  the  metazoa  are  those  which  depart  least  from  the 


i 


82  ORGANISMS  OF  TISSUES 

gastrula  type  of  structure,  while  from  these  to  the  most  complex 
organisms  there  is  every  grade  of  complexity.  Just  as  the  egg 
cell  of  metazoa  is  represented  by  organisms — the  protozoa — 
which  never  go  beyond  this  single  cell  condition,  or  the  blastula 
by  colony  forms  which  do  not  develop  beyond  this  stage,  so  the 
gastrula  is  represented  by  one  group  of  organisms,  termed 
Coelenterata,  which  do  not  develop  beyond  the  gastrula  or  two 
layered  stage  in  the  development  of  metazoa. 

Forming  as  they  do  the  lowest  branch  of  the  metazoan  tree, 
the  coelenterates  demand  particular  attention,  because  through 
them  we  are  introduced  to  many  of  the  essential  problems  in  the 
general  biology  of  higher  animal  forms.  A  good  type  to  begin 
with  is  the  common  fresh  water  Hydra. 

A.  HYDRA  FUSCA  AND  HYDRA  VIRIDIS 

Like  the  majority  of  protozoa,  Hydra  always  lives  in  water, 
and  usually  in  fresh  water  although  some  types  live  in  salt 
water.  They  are  sedentary  forms  attached  by  one  end,  termed 
the  pedal  disc,  to  water  plants  or  other  objects.  The  body  is 
cylindrical,  a  double  wall  of  ectoderm  and  endoderm  enclosing 
one  single  cavity,  the  enteron,  and  terminates  in  a  mouth-bear- 
ing or  oral  end.  The  mouth  is  surrounded  by  a  crown  of  ten- 
tacles which  vary  in  number,  usually  from  five  to  eight  (some 
allied  forms  of  Hydra,  e.g.,  Microhydra  and  Protohydra  have  no 
tentacles).  The  pedal  extremity  is  somewhat  dilated,  forming  a 
sucking  disc  for  attachment  to  foreign  objects.  Thus  attached, 
it  sways  about  with  the  currents  in  the  water,  with  its  ten- 
tacles widely  spread  for  the  capture  of  prey  (Fig.  5,  p.  14). 

Radial  Symmetry. — Because  of  its  cylindrical  body  it  is  pos- 
sible to  cut  Hydra  vertically  through  the  mouth  in  an  infinite 
number  of  planes,  each  of  which  would  result  in  two  symmetrical 
halves.  In  the  majority  of  other  metazoa  only  one  plane,  that 
passing  through  the  mouth  vertically,  will  divide  the  body 
symmetrically;  such  higher  animals  are  bilaterally  symmetrical, 
whereas  Hydra  is  said  to  be  radially  symmetrical.  Radial 
symmetry  in  animals  is  supposed  to  be  due  to  the  fact  that  they 


STRUCTURE  OF  HYDRA  83 

have  lived  as  attached  organisms,  not  moving  from  place  to 
place  in  search  of  food.  As  a  result  of  the  attached  mode  of  life 
radial  symmetry  is  supposed  to  have  developed  because  of 
equal  pressure  on  all  sides.  Another  group  of  organisms, 
the  Echinodermata  (star  fish,  sea-urchins,  sand  dollars,  etc.), 
have  partially  acquired  through  attachment  at  some  time 
and  in  some  past  age,  similar  radial  symmetry,  but  their 
phylogenetic  history  is  far  more  complex  than  that  of  the 
Coelenterata.  Both  cases,  however,  are  excellent  illustrations 
of  the  effect  of  the  mode  of  life  upon  body  forms  of  animals. 

HISTOLOGY 

Several  different  types  of  cells  make  up  the  two  layers,  ecto- 
derm and  endoderm,  of  Hydra.  Excepting  the  reproductive 
cells  these  several  types  are  not  bound  together  into  definite 
aggregates  or  organs,  but  are  distributed  over  the  entire  or- 
ganism, forming  diffuse  tissues.  Between  the  two  layers  is  a 
structureless  and  non-cellular,  gelatinous  intermediate  layer 
termed  the  mesogloea  or  supporting  lamella  (Fig.  34).  The 
mouth  is  at  the  top  of  a  small  rounded  or  conical  prominence, 
called  the  hypostome,  and  lies  in  the  center  of  the  crown  of 
tentacles.  It  opens  directly  into  the  digestive  cavity,  thus 
corresponding  to  the  blastopore  which  opens  into  the  arch- 
enteron  of  the  gastrula.  Hydra  and  the  coelenterates  in  general 
are  often  called  the  diblastic  or  two-layered  animals,  as  dis- 
tinguished from  the  triploblastic  or  three-layered  higher  animals 
made  up  of  ectoderm,  endoderm  and  intermediate  layer,  the 
mesoderm. 

The  different  types  of  cells  of  Hydra  perform  their  functions 
for  the  good  of  the  entire  organism,  and  represent,  morpholog- 
ically, the  incipient  stages  of  organ  systems  in  more  highly  dif- 
ferentiated animals. 

A.  ECTODERM  CELLS. — Six  types  of  cells  are  present  in  the 
ectoderm:  (i)  epithelio-muscle  (neuro-muscle)  cells;  (2)  nettle  or 
stinging  cells;  (3)  nerve  cells;  (4)  sensory  cells;  (5)  germ  cells; 
(6)  formative  or  interstitial  cells.  The  bulk  of  the  body  cover- 


84 


ORGANISMS  OF  TISSUES 


ing  is  made  up  of  the  epithelio-muscle  cells,  while  sensory  cells 
are  rare  and  limited  to  the  regions  about  the  mouth  and  the 
pedal  disc.  The  nettle  or  stinging  cells  are  superficially  placed 
on  the  epithelial  cells  and  partly  embedded  in  them.  The  nerve 
and  interstitial  cells  lie  between  the  bases  of  the  epithelial  cells 
and  upon  the  supporting  lamella. 


FIG.  34. — Hydra  fusca  as  seen  in  optical  section  through  the  enteric  cavity; 
two  testes  are  shown  just  below  the  tentacles  and  an  ovary  farther  down;  on 
the  opposite  side  a  well-developed  bud.  (Modified  after  Marshall  and  Hurst.) 

i.  The  Epithelio-muscle  Cells. — There  is  no  muscular  system 
in  Hydra,  the  covering,  or  epithelial  cells,  possessing  contractile 
processes  which  take  the  part  of  muscles  in  higher  animals. 
These  cells  are  much  elongated  in  the  gonad  region  and  on  the 
pedal  disc  where  the  cells  are  loaded  with  granules  of  secretion 


HISTOLOGY  OF  HYDRA 


85 


(Fig.  35).  On  the  tentacles  they  are  much  flattened,  while  in 
other  regions  of  the  body  their  size  is  intermediate.  The  trunk 
epithelial  cells  are  more  or  less  vacuolated.  All  forms  of  these 
epithelial  cells  are  characterized  by  the  presence  of  muscular 
fibers  (myofibrils)  in  the  basal  parts  of  the  cells.  These  fibers 
run  up  and  down  the  body,  thus  forming  a  complete  longitudinal 
muscular  investment  for  the  entire  animal,  giving  it,  with  the 
transverse  muscle  processes  of  the  endoderm  cells,  the  power  of 
movement  in  all  directions. 

In  addition  to  the  contractile  power,  the  epithelio-muscle  cells 
of  the  pedal  disc  and  of  the  tentacles  have  the  power  of  forming 


FIG.  35. — Epithelio-muscle  cells  from  Hydra  fusca  showing  myofibrils  and  secre- 
tory granules.     (From  Schneider.) 

pseudopodia  by  means  of  which  Hydra  attaches  itself  to  the 
sub-stratum,  alternately  by  tentacles  and  pedal  disc,  and  thus 
moves  from  place  to  place. 

2.  Nettle  or  Stinging  Cells. — These  peculiar  cells  are  typical  of 
coelenterates  and  are  not  found  elsewhere,  although  we  have  seen 
analogous  structures  in  the  trichocysts  of  Infusoria,  They  are 
absent  on  the  pedal  disc  but  are  particularly  abundant  on  the 
tentacles  and  on  the  hypostome,  where  they  are  arranged  in 
groups,  usually  one  large  one  surrounded  by  a  crown  of  smaller 
ones.  They  are  called, nettle  or  stinging  cells  because  of  the 
presence  of  a  coiled  thread  whiclTis~thrown  out  when  the  cell  is 
irritated.  The  tip  of  the  thread  contains  a  trace  of  poison,  so 
that  minute  animals  struck  by  them  are  paralyzed  and  become 
an  easy  prey  for  the  tentacles  and  mouth.  .  These  cells,  which 


86 


ORGANISMS  OF  TISSUES 


are  sometimes  called  nematoblasts  or  cnidoblasts,  thus  perform 
functions  of  offence  and  defence. 

Each  stinging  cell  possesses  a  sensory  hair  or  "trigger," 
called  a  cnidocil,  at  the  free  end,  and  a  thread-holding  capsule, 
the  nematocyst,  within.  The  thread  is  formed  from  a  cell 
growth  which  is  spirally  wound  in  the  capsule,  while  capsule 
and  its  contents  are  all  formed  by  differentiation  of  a  single 
nucleated  cell  of  the  ectoderm.  During  growth  of  the  capsule 
and  thread  the  young  nettle  cells  first  lie  near  the  supporting 


FIG.  36. — Diagrammatic  figure  of  the  cells  from  a  small  portion  of  the  body  wall 
of  Hydra  mridis;  the  ectodermal  cells  with  nematocysts  below  (one  with  pro- 
truded thread),  and  the  large  vacuolated  endoderm  cells  above.  The  symbiotic 
algae  are  grouped  near  the  supporting  lamella  at  the  bases  of  the  endoderm  cells. 
(From  Marshall  and  Hurst.) 

lamella  in  the  region  of  the  mouth,  but  they  migrate  during 
the  period  of  their  formation,  and  finally  come  to  lie  on  the  sur- 
face of  the  ectoderm  around  the  mouth  or  on  the  tentacles  and 
body,  the  cnidocils  ultimately  projecting  slightly  beyond  the 
surface  of  the  body  in  the  surrounding  medium  (Fig.  36). 

3.  Nerve  Cells. — The  nerve  cells  of  Hydra  and  the  coelenter- 
ates  represent  the  special  differentiation  of  cells  for  the  per- 


HISTOLOGY  OF  HYDRA 


87 


formance  of  the  single  function  of  irritability.  All  cells  of 
Hydra,  like  all  protoplasm,  are  irritable  and  respond  to  external 
stimuli,  but  the  nerve  cells  are  especially  adapted  in  this  respect, 
receiving  stimuli  from  the  epithelio-muscle  cells,  from  special 
sensory  cells  or  from  other  nerve  cells,  and  transmitting  them  to 
other  nerve  cells  and  to  the  plexus  of  muscle  fibers  around  the 
organism.  While  more  numerous  in  the  region  about  the  mouth 
they  are  not  combined  into  special  nerve  centers  or  ganglia, 
but,  like  the  muscle  processes,  they  form  an  interlacing  network 
throughout  the  animal. 


FIG.  37. — Plexus  of  nerve  cells  in  the  ectoderm  of  Hydra  fusca;  the  parallel 
lines  represent  the  longitudinal  muscle  fibers  on  the  supporting  lamella.  (From 
K.  C.  Schneider.) 

The  small  cell  bodies  are  either  bipolar  or  multipolar  in 
form,  and  fine  fibers  or  processes  extend  from  the  poles,  often 
for  considerable  distances,  into  the  surrounding  tissues  (Fig. 
37).  These  fibers  are  the  means  of  communication  between 
nerve  cells,  nerve  cells  and  muscle  processes,  and  nerve  cells 
and  nettle  cells,  and  through  their  coordinating  activities  the 
entire  organism  acts  as  a  unit. 

4.  Sensory  Cells. — Special  cells  for  receiving  external  stimuli 
are  much  more  common  in  the  endoderm  than  in  the  ectoderm, 
but  are  found  sparingly  about  the  mouth  and  on  the  pedal 


88  ORGANISMS  OF  TISSUES 

disc.  They  are  fine  thread-like  cells  crowded  in  between  the 
epithelial  cells,  and  run  out  at  the  basal  ends  into  branching 
fibers  which  connect  the  sensory  cells  with  nerve  and  muscle 
cells. 

5.  Reproductive  Cells. — Hydra  is  hermaphrodite,  that  is,  pro- 
vided with  both  male  (spermatozoa)  and  female  (ova)  germ 
cells.     The  former  are  aggregated  into  gonads   called  testes, 
the  latter  into  gonads  called  ovaries.     When  immature,    the 
germ  cells  cannot  be  distinguished  from  other  formative  cells, 
but  when  the  organism  is  mature,  the  germ  cells  accumulate 
between  the  epithelial  cells,  forming  characteristic  swellings 
of  the  male  and  female  gonads.     The  male  gonads,  which  are 
usually  multiple,  develop  as  a  rule  before  the  latter,  and  usually 
in  the  vicinity  of  the  tentacle  bases,  while  the  ovaries  usually 
form  near  the  foot  (Fig.  34).     A  mature  testis  contains  multi- 
tudes of  spermatozoa  which  may  be  seen  in  active  movement 
within  the  gonad;  but  only  one  egg  develops  in  the  ovary. 

6.  Formative  Cells. — The  formative  or  interstitial  cells,  finally, 
are  present  as  minute  rounded  cells  heaped  up  between  the 
epithelial  cells  on  the  supporting  lamella,  and  are  especially 
numerous  on  the  hypostome.     They  form  the  cell  reserves  of 
Hydra,  replacing  nettle  cells  and  nerve  cells  when  exhausted, 
and  give  rise  by  growth  and  division  to  the  reproductive  cells. 
During  regeneration  after  injury  the  new  epithelio-muscle  cells 
are  likewise  derived  from  these  reserves.     They  are,  therefore, 
typical  embryonic  or  generalized  cells  from  which  all  other 
elements  of  Hydra  may  be  replaced. 

B.  THE  ENDODERM. — The  endoderm  of  Hydra,  like  the  ecto- 
derm, is  made  up  of  six  types  of  cells:  (i)  nutritive  muscle  cells; 
(2)  slime  cells;  (3)  albumen  cells;  (4)  sensory  cells;  (5)  nerve  cells; 
and  (6)  formative  cells.  The  nerve  and  formative  cells,  as  in 
the  ectoderm,  lie  between  the  bases  of  the  epithelial  nutritive 
cells,  and  are  comparatively  rare.  The  slime  and  sensory  cells 
are  almost  exclusively  limited  to  the  mouth  region. 

i.  The  Nutritive  Muscle  Cells. — These  are  elongated  cylindri- 
cal cells  somewhat  enlarged  at  the  distal  rounded  ends  which 
may  bear  flagella  (Fig.  36).  The  cytoplasm  is  highly  vacuo- 


HISTOLOGY  OF  HYDRA  89 

lated,  and  is  frequently  loaded  with  food  substances  taken  into 
the  cell  through  the  activity  of  pseudopodia  which  may  also  be 
formed  at  the  free  ends.  In  Hydra  viridis  the  nutritive  cells 
contain  living  algae  (zoochlorellae)  which  give  the  green  color 
to  the  animal.  At  the  basal  ends  the  cells  are  developed  into 
muscle  processes  similar  to  those  of  the  ectodermal  cells,  but 
they  are  usually  finer  and  shorter  and  run  at  right  angles  to  the 
long  axis  of  the  body.  The  contraction  of  these  muscle  processes 
lengthens  the  animal  while  contraction  of  the  longitudinal 
muscle  processes  shortens  it. 

2.  Slime  Cells. — These  are  very  numerous  in  the  region  of 
the  mouth  and  gullet  where,  as  short  cylindrical  cells,  they  lie 
between  the  nutritive  muscle  cells  and  usually  at  some  distance 
from   the  supporting  lamella.     They  are  usually  filled  with 
granules  which  on  discharge  from  the  broader  distal  end  form 
a  slimy  secretion  surrounding  the  prey  taken  in  as  food. 

3.  Albumen  Cells. — These  are  similar  to  the  slime-forming 
cells  but  are  more  widely  distributed,  and  each  cell  is  drawn  out  at 
the  base  in  a  thin  process  which  rests  on  the  supporting  lamella. 
At  the  opposite  free  end  they  have,  like  the  nutritive  cells,  two 
or  sometimes  three  long  flagella.     The  large  granules  of  secre- 
tion usually  lie  in  vacuoles. 

4.  Sensory  Cells. — These  fine  thread-like  cells  are  much  more 
numerous  here  than  in  the  ectoderm.     They  may  bear  two 
fiagella,  like  nutritive  cells,  or  only  one  flagellum,  while  oc- 
casionally there  is  none  at  all.     The  basal  ends  of  these  cells 
are  prolonged  into  fine  branching  nerve  processes  which  form, 
with  the  nerve  fibers,  a  plexus  on  the  inner  layer  of  transverse 
muscle  processes. 

5.  Nerve  Cells. — These  agree  throughout  with  the  ectodermal 
nerve  cells,  and  while  less  numerous  are  distributed  in  much  the 
same  way,  but  are  apparently  absent  from  the  endodermal 
cell  of  the  tentacles. 

6.  The  Formative  Cells. — These  also  are  less  numerous  than 
in  the  ectoderm,  their  chief  reparative  activity  apparently  being 
the  new  formation  of  albumen  cells,  since  nettle  cells  and  re- 
productive cells  are  absent.     In  regeneration  after  injury,  how- 


90  ORGANISMS  OF  TISSUES 

ever,  they  give  rise,  as  in  the  ectoderm,  to  the  various  cells  of  the 
inner  layer. 

C.  THE  SUPPORTING  LAMELLA. — The  mesogloea,  separating 
ectoderm  and  endoderm  and  broken  only  at  the  mouth,  is  the 
only  supporting  structure  of  the  organism.  It  is  derived  from 
both  layers  by  secretion  from  the  cells  which  abut  against 
it.  It  is  homogeneous  throughout,  and  the  muscle  processes 
are  slightly  embedded  in  it. 

PHYSIOLOGY 

In  its  functional  activities  Hydra  stands  midway  between 
the  unicellular  protozoa  and  metazoa  with  well  denned  organs. 
In  protozoa  the  protoplasm  is  differentiated  for  different  func- 
tions, the  ectoplasm  for  locomotion  and  food  getting,  the  endo- 
plasm  for  digestion  and  assimilation  and  for  the  elaboration  of 
various  parts  of  the  cell.  Hydra  is  made  up  of  multitudes 
of  cells,  most  of  which  are  physiologically  unbalanced,  that  is, 
some  one  function  predominates  over  all  others.  Of  these  there 
are  eight  distinct  types,  while  one  additional  type — the  forma- 
tive cells — is  physiologically  balanced.  These  nine  types  of 
cells  do  not  get  beyond  the  tissue  stage  in  differentiation  and 
internal  organs  or  aggregates  of  like  cells,  and  tissues  for  per- 
forming special  vital  functions  are  not  developed.  Some  ad- 
vance in  this  direction,  however,  is  seen  in  the  tentacles,  the 
mouth  and  hypostome,  and  the  gonads. 

While  the  apparatus  for  performing  them  is  relatively  simple 
the  vital  activities  of  Hydra  are  exactly  the  same  as  in  other 
living  animals,  and  may  be  grouped  under  the  headings:  (a) 
nutrition;  (b)  excretion;  (c)  respiration;  (d)  irritability  and  (e) 
reproduction. 

v  A.  NUTRITION. — The  food  of  Hydra  consists  of  any  minute 
living  thing  in  the  surrounding  water  but  it  seems  to  be  par- 
ticularly fond  of  small  Crustacea  and  embryos  of  various 
water-dwelling  animals.  A  passing  Cypris  touches  a  tentacle 
and  is  stung  by  the  poisoned  threads  of  the  nettle  cells;  this 
poison,  called  "hypnotoxin,"  paralyzes  the  prey  while  the 


DIGESTION  IN  HYDRA  91 

threads  anchor  it  to  the  captor.  Other  tentacles  turn  to  it, 
and  it  is  passively  drawn  to  the  mouth  and  swallowed.  It 
enters  the  large  sac-like  enteron  where  digestion  takes  place. 
This  is  accomplished  by  a  combination  of  methods,  of  higher 
animals  and  of  protozoa.  As  in  the  stomach  of  a  higher  animal 
digestive  ferments  are  poured  into  the  digestive  cavity  from  the 
gland  cells.  The  slime  cells,  as  shown  above,  are  limited  to  the 
oral  region,  and  no  gland  cells  are  on  the  pedal  disc.  The  prey, 
apparently  in  accord  with  this  distribution,  is  kept  in  the  upper 
region  of  the  enteron  and  in  the  vicinity  of  the  mouth.  There 
is  some  uncertainty  as  to  the  nature  of  the  ferment  in  Hydra 
but  in  distant  related  coelenterates  (Actinians)  its  reactions  are 
similar  to  those  of  trypsin.  Starch-dissolving  ferments  are 
absent,  and  starch  grains  fed  to  Hydra  are  thrown  out  unaltered. 

The  result  of  ferment  activity  is  the  dissolution  of  the  prey 
into  soluble  substances  and  fine  particles  of  solid  matter,  and  it 
is  in  connection  with  the  latter  that  the  protozoon  method  of 
digestion  is  involved.  The  small  solid  particles  are  seized  by 
pseudopodia  formed  by  the  nutritive  muscle  cells  and  drawn 
into  the  protoplasm  of  these  cells.  Here  intra-cellular  digestion 
takes  place  exactly  as  in  an  Amoeba  or  allied  form.  Such  seizure 
and  intra-cellular  digestion  is  called  phagocytosis  and  the  cells, 
because  of  this  function,  are  known  as  phagocytes.  Similar 
functions  are  performed  by  white  blood  cells  (phagocytes)  of 
higher  animals,  but  not  in  connection  with  metabolism  (see 
p.  200).  Nothing  is  known  about  the  process  of  absorption  of 
the  digested  food;  presumably  it  takes  place  by  absorption  from 
cell  to  cell  since  there  is  no  blood  system  to  carry  the  products 
of  digestion  to  different  parts  of  the  organism. 

The  undigested  food  substances  like  starch,  cellulose,  chitin, 
etc.,  are  defecated  through  the  mouth,  so  this  organ  acts  both 
as  mouth  and  anus.  In  this  respect  many  of  the  protozoa 
(ciliates)  are  more  specialized  than  Hydra  in  having  a  definite 
anal  opening  quite  as  distinct  as  the  mouth  and,  in  some  cases, 
with  special  cilia  to  aid  in  defecation. 

In  animals  higher  in  the  scale  than  Hydra  the  digestive  sys- 
tem gradually  becomes  more  and  more  perfected.  In  the  round 


92  ORGANISMS  OF  TISSUES 

worms  and  annelids  it  becomes  a  long  tube  with  mouth  at  one 
end  and  anus  at  the  other.  After  this  the  chief  advance  is  in 
the  concentration  of  different  cell  types  and  specialization  of 
the  tube  into  receptacles  and  digestive  glands.  The  mouth 
becomes  an  organ  with  jaws  and  teeth  for  masticating  food,  and 
with  salivary  glands  to  moisten  it;  through  a  gullet  the  food 
passes  to  a  single  or  multiple  digestive  viscus,  the  stomach, 
and  thence  through  a  long  intestine  where  the  products  of 
digestion  are  usually  absorbed  into  the  blood  vascular  system. 
Intra-cellular  digestion  ceases  entirely;  the  cells  instead  are 
differentiated  into  various  kinds  of  ferment  producers  whose 
secretions  are  poured  into  the  digestive  tract  and  combine  to 
make  all  kinds  of  food, — proteins,  fats,  and  carbohydrates, 
ready  for  absorption  through  the  walls  of  the  intestine.  Gradu- 
ally the  digestive  organs  acquire  a  close  relation  with  the 
excretory  organs,  so  that  useless  or  damaging  products  of  diges- 
tion may  be  removed  in  the  liver  from  the  loaded  blood  before 
distribution  to  the  general  system.  In  this  way  an  organ  sys- 
tem is  slowly  evolved  from  a  primitive  digestion  apparatus  like 
that  of  Hydra  with  its  protozoa-like  peculiarities.  Similarly 
with  other  organ  systems,  specialization  and  continued  division 
of  labor  result  in  the  complex  aggregates  of  cells,  tissues  and 
organs  which  we  know  in  the  higher  animal  types. 

B.  EXCRETION  AND  RESPIRATION. — Practically  all  cells  of 
Hydra  are  in  contact  with  the  surrounding  water,  the  ectoder- 
mal  cells  directly  with  the  surrounding  medium,  the  endodermal 
system  with  water  taken  in  with  food.    As  with  protozoa,  no 
special  organs  are  necessary  for  excretion  and  respiration  but 
the  waste  matters  of  metabolism  are  given  off  directly  by 
transfusion  from  the  cells,  and  oxygen  is  absorbed  by  the 
protoplasm  from  the  water.     In  respiration  this  is  exactly  the 
manner  in  which  higher  animals  get  their  oxygen;  the  only 
difference  is  that  the  cells  of  hydra  absorb  it  from  the  medium 
water  while  the  cells  of  higher  animals  absorb  it  from  the  me- 
dium blood  or  the  medium  air  (insects  and  other  tracheates). 

C.  IRRITABILITY — Hydra   reacts  to  external  stimuli  of  all 
kinds  by  contraction  of  the  muscle  processes  of  the  epithelial 


IRRITABILITY  OF  HYDRA  93 

cells.  Contraction  of  the  ectodermal  cells  results  in  shortening 
of  the  body  or  in  local  movements  of  the  tentacles.  The 
stimuli  are  received  by  either  the  sensory  cells  or  by  the  ecto- 
dermal epithelio-muscle  cells,  a  function  which  has  led  to  the 
name  neuro-muscle  cells  sometimes  applied  to  them.  The 
impulse  thus  received  is  transmitted  to  the  nerve  cells  especially 
endowed  with  the  power  of  transmission,  and  a  general  co- 
ordinated reaction  follows. 

The  Nervous  System. — It  is  important  to  note  that  with  the 
nervous  system  a  delicate  function  of  protoplasm,  irritability, 
is  singled  out  and  made  the  special  function  of  a  complicated 
series  of  cells  and  fibers.  It  is  the  centralizing  and  unifying 
system  of  the  organism,  whereby  the  most  widely  separated 
parts  of  the  individual  are  made  to  act  in  harmony  for  the  cap- 
ture of  food  or  escape  from  enemies.  No  such  specialization 
is  found  in  protozoa — apparently  there  is  no  need  for  it  since 
the  single  cell  must  act  as  a  whole.  In  Hydra  there  is  such  need, 
but  we  find  that  the  nervous  system  or  apparatus  for  co-ordinat- 
ing muscular  actions  is  extremely  simple,  consisting  of  a  nerve- 
cell  plexus  enveloping  the  whole  animal.  There  is  no  evidence 
of  a  centralized  nervous  system  to  which  impulses  due  to  ex- 
ternal stimuli  are  sent  and  from  which  motor  impulses  to  muscle 
groups  are  transmitted.  This  comes  first  in  higher  forms,  and  is 
accomplished  by  aggregation  of  nerve  cells  into  ganglia  of  the 
central  nervous  system.  One  part  of  this  system,  the  brain, 
with  further  advance  and  specialization,  becomes  the  special 
center  for  reception,  analysis  of  external  and  internal  stimuli  and 
for  utilization  and^co^ojrdination  of  multifarious  impressions  and 
responses,  functions  which  we  associate  together  under  the 
head  of  consciousness.  The  term  loses  its  meaning  when  used 
to  describe  reactions  of  animals  in  which  the  co-ordinating 
center  has  notl  eadied  a  cer  tain  degree  ot  complexity,  but  there 
are  undoubtedly  different  grades  or  degrees  of  consciousness, 
just  as  there  are  different  grades  of  complexity  in  the  nervous 
system. 

With  Hydra  there  is  no  such  centralization,  but  a  step 
in  this  direction  is  seen  in  Hydra  and  in  the  higher  types  of 


94 


ORGANISMS  OF  TISSUES 


coelenterates,  in  the  accumulation  of  nerve  cells  around  the 
mouth  which  thus  becomes  the  most  sensitive  or  irritable  part 
of  the  body. 


D 


FIG.  38. — The  egg  of  Hydra  and  its  development.  A ,  The  mature  ovum  full  of 
yolk  granules;  B,  section  of  blastula  formed  by  segmentation  of  the  ovum;  C, 
formation  of  the  inner  mass  of  cells  by  transverse  division  and  immigration  of 
the  outer  cells;  D,  solid  mass  of  endoderm  and  ectoderm  cells,  and  cyst-like 
outer  membrane;  F,  embryo  emerging  from  membrane;  E,  discarded  mem- 
brane; and  G,  separation  of  endodermal  cells  to  form  the  enteric  cavity. 
(From  Dendy,  after  Bourne  and  Brauer.) 

D.  REPRODUCTION. —The  naturalist  Trembley,  living  in  the 
i8th  century,  discovered  that  a  Hydra  might  be  cut  into  many 
pieces,  and  that  each  piece  would  continue  to  live  and  would 


REPRODUCTION  OF  HYDRA  95 

develop  into  a  complete  Hydra.  This  power  of  regeneration,  or 
re-growth  of  the  whole  animal  from  a  part,  is  a  characteristic 
of  all  of  the  lower  animals  and  is  an  evidence  of  their  generalized 
character.  Some  forms  of  the  lower  animals  (certain  worms 
for  example)  actually  reproduce  by  spontaneously  breaking  into 
pieces.  Hydra  does  not  reproduce  in  this  way  but,  in  addition 
to  sexual  reproduction,  has  a  more  simple  method  of  propagating 
its  kind,  namely,  by  budding.  At  certain  times,  on  healthy 
well-nourished  Hydras,  swellings  appear  near  the  foot.  Such  a 
swelling,  which  involves  both  layers  ectoderm  and  endoderm, 
soon  takes  the  form  of  a  young  Hydra,  mouth  and  tentacles 
appearing  on  the  distal  end  and  the  trunk  growing  until  it  has 
nearly  the  length  of  the  parent  organism.  The  bud  or  young 
Hydra  then  separates  from  the  parent  by  constriction  of  the 
foot  and  starts  on  an  independent  career  (Fig.  5) . 

Among  the  cells  of  Hydra,  distributed  here  and  there  in  the 
tissues,  are  male  and  female  germ  cells  which  collect  from  time 
to  time  in  definite  regions  to  form  the  gonads.  Of  these  the 
testes  or  male  organs  are  formed  near  the  base  of  the  tentacles, 
while  the  female  organ  or  ovary  is  formed  nearer  the  pedal 
disc. 

The  spermatozoa  are  developed  from  the  formative  or  inter- 
stitial cells  of  the  ectoderm,  and  collect  in  large  numbers  be- 
tween the  epithelio-muscle  cells.  They  do  not  change  directly 
into  spermatozoa,  but  each  one  is  a  primordial  sperm  cell  which 
divides  two  or  more  times  before  transforming  into  sperma- 
tozoa.  When  mature  the  spermatozoa  are  liberated  by  rupture 
of  the  outer  wallsTof  the  testis,  and  live  for  a  longer  or  shorter 
period  in  the  water  until  they  come  in  contact  with  egg  cells; 
otherwise,  they  die. 

There  is  usually  only  a  single  ovary,  and  in  the  ovary  only  a 
single  egg.  But  the  female  gonad,  like  the  testes,  starts  with 
an  accumulation  of  formative  cells  which  divide  and  produce  a 
number  of  potential  egg  cells.  Only  one  develops,  however,  the 
others  being  devoured  by  the  successful  egg,  which,  as  an  amoe- 
boid cell,  puts  out  pseudopodia  and  feeds  upon  the  sister  cells 
(Fig.  38,  A).  The  ovum  grows  to  a  relatively  large  size  and  be- 


96  ORGANISMS  OF  TISSUES 

comes  loaded  with  yolk  bodies;  the  external  cells  of  the  ovary 
finally  break  away,  leaving  the  ovum  exposed.  A  spermatozoon 
bores  its  way  into  the  egg  and  fertilizes  it. 

Development  begins  while  the  egg  is  still  attached  to  the 
parent  Hydra.  A  hollow  ball  of  cells,  or  bias  tula,  is  formed, 
consisting  of  a  single  layer  of  cells  enclosing  the  segmentation 
cavity  (Fig.  38,  B).  Cells  then  begin  to  migrate  from  the  pe- 
riphery into  the  segmentation  cavity  which  ultimately  is  filled 
by  the  mass  of  hypoblast  cells,  thus  forming  a  solid  endoderm 
(Fig.  38,  C,  D).  The  outer  layer  of  cells,  ectoblast,  secretes  a 
horny  protective  envelope  about  the  embryo,  which  now  leaves 
the  parent  and  rests  without  further  development  for  some 
time  on  the  bottom  of  the  pond.  Finally  development  begins 
again;  the  cyst  or  shell  is  ruptured,  and  the  embryo  escapes. 
The  endoderm  cells  begin  to  separate  from  one  another,  leaving 
a  hollow  in  the  middle;  this  is  the  enteric  cavity  from  which 
a  mouth  breaks  through,  and  tentacles  develop  around  it  (Fig. 
38,  F).  The  gastrula  stage,  or  two-layered  pouch  with  blasto- 
pore,  is  attained,  with  the  formation  of  the  mouth,  but  the 
method  of  gastrulation  differs  from  that  described  on  p.  80. 
The  same  result  is  reached,  in  typical  development  by  in- 
vagination,  here  by  inwandering  or  immigration  of  ectoblastic 
cells. 

SYMBIOSIS 

In  addition  to  Microhydra  (a  form  without  tentacles)  and 
Hydra  fusca  (brown  Hydra),  there  is  another  and  an  equally 
common  species  of  fresh  water  forms — Hydra  mridis.  As  the 
specific  name  indicates,  this  Hydra  is  colored  green,  the  color 
being  due  to  the  presence  of  minute  unicellular  plants,  Chlorella 
vulgaris.  The  plant  cells  are  situated  in  the  basal  parts  of  the 
nutritive  muscle  cells  where  they  form  an  almost  complete 
sheath  of  investing  plant  organisms,  the  cells  of  the  ends  of  the 
tentacles  alone  being  free  from  them.  Buds,  when  formed, 
include  the  accompanying  plants  and  are  quite  as  green  as  the 
parent. 


SYMBIOSIS  97 

The  chief  interest,  biologically,  of  these  green  cells  lies  in  their 
physiological  relations  to  Hydra.  They  are  not  parasites, 
for  parasites  are  organisms  living  at  the  expense  of  other 
organisms  to  which  they  are  detrimental,  either  structurally 
or  functionally.  Hydra  is  not  inconvenienced  in  any  way  by 
the  presence  of  these  green  cells,  but  lives,  grows,  and  reproduces 
normally.  On  the  contrary  there  is  every  reason  to  believe 
that  the  foreign  cells  are  beneficial  to  Hydra,  for  green  plants 
are  essentially  constructive  in  their  vital  activities  and  make 
use  of  CO2  for  the  construction  of  starch.  This  CO2  is  a  waste 
product  of  metabolism  of  the  Hydra  cells,  and  the  green  cells 
are  of  service  in  freeing  them  of  it;  thus  Hydra  mridis  will  live 
much  longer  in  an  atmosphere  of  C02  than  Hydra  fusca  (Hadzi). 
Furthermore  the  active  green  cells  give  off  oxygen  which  the 
Hydra  cells  need.  Hydra  viridis  may  be  freed  from  its  accom- 
panying plant  guests  by  being  kept  in  dilute  glycerine,  and  will 
continue  to  live  as  a  white  Hydra  although  it  does  not  have  the 
same  vitality,  and  its  growth  and  reproduction  are  much  slower 
(Whitney).  It  is  probable,  therefore,  that  the  green  cells  are 
beneficial  in  supplementing  the  normal  metabolic  processes  of 
Hydra. 

Just  what  advantages  the.  green  cells  obtain  from  this 
peculiar  habitat  are  less  obvious.  They  are  protected  by  the 
animal  and  probably  receive  a  constant  supply  of  CO2.  Out 
of  the  sunlight,  as  at  night,  they  require  oxygen  and  give  off 
CO2  just  as  animal  cells  do;  the  oxygen  at  such  times  may  be 
obtained  from  the  surrounding  water  by  diffusion  in  the  same 
way  that  the  Hydra  cells  obtain  it,  while  the  CO2  and  other 
waste  matters  are  probably  disposed  of  in  the  same  way  as  in 
the  case  of  Hydra  fusca,  by  osmosis.  On  the  whole,  therefore, 
there  is  some  gain  to  both  types  of  organisms  by  this  joint  house- 
keeping, the  advantages,  which  we  can  only  guess  at,  being 
proved  by  their  thriving  vitality. 

This  phenomenon  of  living  together  is  termed  symbiosis 
or  messmateism,  and  differs  from  parasitism  in  the  fact  that  no 
structural  or  functional  harm  is  done  to  either  organism,  host 
and  guests  living  together  for  mutual  benefit. 


98  ORGANISMS  OF  TISSUES 

POLYMORPHISM.       COLONY   FORMATION    AND    INDIVIDUAL 
DIFFERENTIATION 

The  individual  Hydra  fusca  in  its  relation  to  other  individuals 
may  be  compared  with  an  Amoeba  or  Paramecium  where,  after 
division,  the  daughter  individuals  separate  and  live  independ- 
ently. There  are  many  species  of  Coelenterates,  however, 
more  or  less  closely  related  to  Hydra,  in  which  the  individuals, 
after  their  formation  by  budding,  remain  attached  to  the 
parent  organism,  thus  forming  colonies  similar  in  origin  and 
mode  of  growth  to  many  colonies  of  protozoa.  Obelia  is 
such  a  colony  form,  the  adult  being  a  graceful,  much  branched 
aggregate  of  individuals,  resembling  a  small  bush  in  its  ap- 
pearance and  mode  of  growth,  a  fact  which  led  Aristotle  to 
name  such  organisms  Zoophyta  or  animal  plants.  Obelia 
begins  as  a  small  hydroid  (Hydra-like),  one-sixty-fourth  to  one- 
thirty-second  of  an  inch  in  height  with  mouth  and  tentacles; 
it  reproduces  by  budding,  while  stalks  or  stems  are  formed  by 
the  secretion  of  a  nitrogenous  material,  chitin.  New  buds  are 
continually  added  to  the  colony,  until  the  latter  attains  the 
height  of  several  inches  and  consists  of  thousands  of  individuals 
with  main  stems  and  side  branches  enclosed  within  chitin, 
the  entire  colony  being  connected  by  living  substance. 

Such  hydroid  colonies  differ  from  colonies  of  protozoa  in  that 
some  individuals  may  become  differentiated  for  the  performance 
of  different  functions.  This  phenomenon,  termed  polymor- 
phism, is  represented  by  Obelia  in  a  relatively  simple  form. 
The  only  differentiation  here  is  the  formation  of  reproductive  in- 
dividuals distinct  from  the  nutritive  individuals.  When  the 
colony  is  mature,  certain  individuals  produce  buds  which  differ 
from  the  ordinary  hydroid  buds  in  structure.  These  buds,  in- 
stead of  remaining  attached  as  do  the  hydroids,  are  detached 
and  swim  away  as  "medusae"  or  jelly  fish.  The  jelly  fish 
bear  the  gonads  (testes  or  ovaries),  and  are  the  sexually  dif- 
ferentiated individuals  of  the  colony.  When  eggs  and  sperma- 
tozoa are  ripe,  they  are  discharged  into  the  water  where  fer- 
tilization takes  place,  the  fertilized  egg  developing  into  a  young 


POLYMORPHISM 


99 


hydroid   which   begins    the    formation   of    a    new    colony    by 

budding.     A  hydroid,  thus,  may  be  polymorphic;  Hydra  is  not. 

Other  types  of  hydroids  present  different  grades  in  complexity 

of  this  phenomenon  of  polymorphism,  some  individuals  being 


FIG.  39. — Stephalia  corona.  A  siphonophore  colony  illustrating  polymorphism. 
A,  aurophore,  a  modified  medusa;  G,  secondary  gastrozooid  or  feeding  polyp; 
T,  tentacle  of  gastrozooid;  PG,  primary  gastrozooid,  N,  nectophore,  one  of  the 
locomotor  individuals  of  the  colony  and  with  the  general  form  of  a  medusa;  P, 
pneumatophore  or  air  sac.  (From  Lankester  after  Haeckel.) 

differentiated  for  purposes  of  locomotion,  others  for  protection 
and  defence,  and  others  for  reproduction.  The  highest  devel- 
opment of  this  coelenterate  polymorphism  is  found  in  the  group 


100  ORGANISMS  OF  TISSUES 

of  Siphonophora,  where  no  less  than  seven  different  kinds 
of  individuals  may  be  present  in  the  same  colony,  all  working 
for  the  common  good  and  each  dependent  on  the  others  for 
some  vital  function  (Fig.  39).  We  have  in  forms  like  Stephalia 
and  the  Portuguese  Man-of-War,  therefore,  a  distinct  type  of 
individual  composed  of  many  individuals,  originally  of  a  com- 
mon type.  Such  forms  have  been  termed  individuals  of  a 
second  order,  and  their  evolution  may  have  been  a  parallel  of 
the  evolution  of  metazoa  where  cells  with  different  functions, 
working  for  the  good  of  the  whole,  were  gradually  evolved 
from  colonies  of  protozoa  in  which  the  cells  are  of  one  type. 

One  other  phenomenon  of  general  biological  importance  was 
briefly  mentioned  above  in  describing  the  sexual  reproduction 
of  Obelia.  The  jelly  fish  or  medusa  produces  eggs  which, 
when  fertilized,  develop  not  into  medusae  but  into  hydroids 
which  produce  other  hydroids  by  asexual  reproduction.  This 
phenomenon,  termed  alternation  of  generations  or  "  metagene- 
sis, "  is  not  often  met  with  in  animals  but  it  is  widely  spread  in 
the  plant  kingdom,  an  asexual  generation  giving  rise  to  a  sexual 
generation  and  this,  in  turn,  to  an  asexual  in  regular  alternation. 

B.  SUMMARY  OF  POINTS  OF  GENERAL  BIOLOGICAL  INTEREST 
SHOWN  BY  HYDRA 

Hydra  and  its  allies  are  important  to  the  student  of  biology 
because  of  their  intermediate  position  between  organisms  com- 
posed of  one  cell  and  organisms  composed  of  organs.  One 
essential  characteristic  of  the  metazoa  is  represented  by  them 
in  the  differentiation  of  cells  for  the  performance  of  the  different 
vital  functions.  Nerve  and  sensory  cells,  muscular  and  support- 
ing cells,  nematoblasts  and  digestive  cells  are  physiologically 
unbalanced,  in  that  they  exhibit  some  one  function  which  pre- 
dominates over  all  of  the  others.  In  Hydra,  at  least,  these 
differentiated  cells  are  not  bound  together  into  specialized  and 
centralized  organs  which  perform  certain  functions  for  the  entire 
individual,  but  are  more  or  less  uniformly  distributed  over  the 
body.  Thus  the  nervous  system  is  diffuse,  that  is,  it  consists  of 


SUMMARY  ;  101 

external  nerve  and  sensory  cells  with  their  long  fibers  or  cell 
processes  which  inter-cross  with  one  another  and  with  muscle 
cells.  In  higher  animals,  similar  nerve  cells  come  together  in 
groups  to  form  ganglia,  and  these  ganglia  are  aggregated  into 
more  or  less  complicated  central  nervous  and  peripheral  sensory 
systems. 

Similarly  with  the  muscle  cells  of  Hydra;  they  are  not  bound 
together  in  complicated  muscle  bundles,  but,  like  nerve  cells,  are 
distributed  over  the  entire  organism,  and  are  connected  only  in  a 
primitive  way  with,  the  nerve  fibers  by  which  they  are  stimulated 
to  contract.  Histologically,  therefore,  Hydra  differs  from 
higher  animals  and  perhaps  indicates  the  generalized  type  of 
structures  from  which  higher  animals  have  been  derived. 

While  Hydra  itself  does  not  show  the  concentration  of  cells 
into  definite  specialized  organs,  we  do  find  types  of  coelenter- 
ates,  especially  the  siphonophores,  where  specialized  organs  are 
developed,  but  in  quite  a  different  way  from  organ  formation  in 
higher  animals.  Polymorphism,  a  second  feature  of  general 
interest,  is  a  form  of  organ  development  in  which  individuals 
themselves  are  modified  as  organs  for  the  performance  of  some 
one  chief  function.  Medusae  are  reproductive  in  function,  and 
are  special  organs  for  sex-cell  formation  and  distribution,  pro- 
duced on  blastostyles  or  reproducing  individuals.  Dactylo- 
zooids  are  individuals  specialized  for  purposes  of  offence  and 
defence.  Gastrozoids  are  feeding  individuals,  and  Necto- 
phores  or  swimming  bells  are  individuals  specialized  for  locomo- 
tion. Each  specialized  individual  performs  its  particular  func- 
tion for  the  good  of  the  whole  colony,  and  their  colony  organiza- 
tion has  led  to  the  descriptive  phrase,  "individuals  of  a  second 
order." 

A  third  feature  of  general  biological  interest  is  an  outcome  of 
these  individualized  organs.  The  medusa  becomes  free-living 
as  an  individual  of  the  sexual  generation;  it  forms  eggs  or 
spermatozoa  (the  sexes  are  separate),  and  the  fertilized  eggs 
develop  into  Hydra-like  asexual  individuals,  called  hydroids, 
which  reproduce  by  budding.  This  phenomenon  is  called 
metagenesis  or  alternation  of  generations,  since  one  (sexual) 


102  ORGANISMS  OF  TISSUES 

generation  (medusa)  alternates  with  another  (asexual)  genera- 
tion (hydroid). 

Other  features  of  general  interest  mentioned  under  different 
headings  above  may  be  summarized  as  (4)  the  bilamellate  or 
diblastic  structure  of  Hydra  representing  a  permanent  gastrula; 
(5)  symbiosis  or  living  together  of  two  different  types  of  organ- 
isms for  mutual  benefit;  and  (6)  the  combination  of  extra-cellu- 
lar and  intracellular  digestion,  thus  combining  the  digestive 
processes  of  higher  metazoa  with  those  of  the  protozoa. 


CHAPTER  V 

PLANTS,  THE  FOOD  OF  ANIMALS  AND  THE  SOURCES 
OF  ANIMAL  ENERGY 

HELMHOLZ,  when  he  outlined  the  great  theory  of  the  con- 
servation of  energy,  is  said  to  have  been  widely  criticized  for 
his  "play  of  fancy"  in  believing  that  all  living  things  get  their 
energy  from  the  sun,  transforming  it  into  vital  activity.  Today 
this  "fancy,'7  refined  through  thousands  of  experiments  on  the 
physiology  of  animals  and  plants,  is  an  accepted  fact.  It  is 
known  that  not  all  living  things  can  use  the  solar  energy  di- 
rectly, plants  alone  having  this  power,  while  animals  obtain 
it  indirectly  through  the  vegetable  world.  The  importance, 
therefore,  of  the  plant  organism  in  general  biology  cannot  be 
overestimated.  In  the  present  chapter  we  will  trace  back  this 
energy  through  the  food  of  animals  to  its  ultimate  source. 

A.  THE  FOOD  OF  ANIMALS 

Hydra,  as  we  have  seen,  like  Paramecium  or  Amoeba,  takes 
in  solid  food  in  the  living  state,  digests  and  assimilates  it.  The 
protoplasmic  molecules  throughout  select  from  the  dissolved 
proteins  the  elements  needed  for  their  reconstruction.  This 
food,  rich  in  potential  energy,  consists  mainly  of  minute  crus- 
tacea,  rotifers,  protozoa,  unicellular  plants  and  bacteria.  The 
potential  energy  carried  over  by  these  organisms  must  in  turn 
be  traced  back  to  their  food.  The  Crustacea,  rotifers,  protozoa, 
etc.,  are  animals,  and  live  upon  solid  living  materials  as  Hydra 
does,  but  being  minute,  their  food  must  be  correspondingly 
small.  Could  Hydra  be  cut  up  in  small  enough  pieces,  minute 
Crustacea,  rotifers  and  protozoa  might  equally  well  get  their 
nourishment  from  its  protein,  a  type  of  retaliation  with  which 
Huxley  has  made  us  familiar  in  the  possible  mutual  gastric 
relations  of  man  and  lobsters. 

103 


104  PLANTS,  THE  FOOD  OF  ANIMALS 

We  have  seen  that  Paramecium  and  perhaps  the  majority 
of  protozoa  live  on  bacteria,  extracting  from  these  minute  cells 
the  elements  necessary  for  their  protoplasmic  reconstruction 
and  growth.  Rotifers  and  Crustacea  feed  upon  the  larvae 
of  animals,  on  smaller  Crustacea  and  rotifers,  and  upon  uni- 
cellular animals  and  plants  such  as  protozoa,  diatoms,  desmids 
and  other  algae.  These  Crustacea,  rotifers  and  larvae  live 
on  unicellular  animals  and  plants.  The  food  of  Hydra,  there- 
fore, traced  back  upon  any  line,  finally  brings  us  to  the 
minute  chlorophyll-bearing  plants  and  bacteria.  These  ulti- 
mate food  materials  of  Hydra,  therefore,  are  living  things  some 
of  which  have  the  power  to  manufacture  their  nutriment  from 
simple  elements  with  the  aid  of  the  sun,  while  the  others  live 
upon  dissolved  protein  matters  from  decomposing  animal  and 
plant  tissues,  or  else  utilize  waste  matters  like  urea  and  rela- 
tively simple  chemical  compounds  for  their  sources  of  energy. 

The  higher  animals,  like  Hydra,  must  likewise  turn  to  the 
plant  world  for  food,  but  to  trace  back  the  connection  in  some 
cases  would  involve  a  far  more  complicated  chain  of  organisms 
than  in  the  case  of  the  simple  coelenterate.  Man  is  almost 
omnivorous,  taking  his  food  directly  from  the  vegetable  king- 
dom in  his  green  vegetables,  cereals,  etc.,  and  from  the  animal  in 
his  beef,  mutton,  fish,  or  fowl.  Cattle,  sheep  and  birds,  in  turn, 
are  herbivorous  or  graminivorous  and  get  their  main  nourish- 
ment from  plants.  Birds,  indeed,  eat  worms  and  insects  to  a 
great  extent  but  worms  and  insects  feed  upon  leaves  and  other 
vegetable  matter,  so  this  food  is  only  one  step  removed  from 
green  matter.  Man  eats  fish,  oysters  and  other  marine  food; 
the  larger  fish  eat  smaller  ones,  these  still  smaller  and  so  on  until 
the  smallest  eat  Crustacea,  larvae,  protozoa  and  other  micro- 
organisms which,  like  rotifers,  subsist  finally  on  microscopic 
algae  and  bacteria.  Carnivorous  animals  live  almost  entirely 
without  green  plants  or  their  products,  and  with  them  the 
ultimate  plant-eating  forms  serving  as  food  are  still  more  remote; 
in  the  end,  however,  we  find  the  same  dependence  upon  the 
vegetable  world  for  proteins  and  potential  energy. 

The  work  of  Hydra  and  higher  animals  is  done  at  the  expense 


FOOD-GETTING  AND  DIFFERENTIATION          105 

of  energy  which  is  transformed  from  the  stored-up  energy  in 
proteins  and  other  foods,  which  in  turn  is  derived  from  the 
energy  of  foods  obtained  from  other  animals  or  plants,  where  in 
turn  it  is  obtained  from  other  foods,  until  finally  this  energy  is 
traced  back  to  the  green  plants.  Green  plants,  in  turn,  get 
their  energy  from  the  sun.  A  necessary  step,  therefore,  in  the 
biology  of  animals  is  to  trace  out  as  far  as  possible  the  transfor- 
mation of  solar  energy  into  that  of  protoplasm.  The  initial 
steps  in  the  process  are  connected  with  the  structures  and  func- 
tions of  plants,  and  enough  of  these  will  be  described  to  pave  the 
way  for  a  clear  understanding  of  the  work  which  the  plants  do 
in  nature. 

The  lines  of  development  of  the  two  great  kingdoms  are 
widely  different.  While  at  bottom  the  vital  activities  of  plants 
and  animals  are  the  same,  the  structural  differentiations  have 
followed  lines  of  entirely  different  requirements.  The  activities 
of  animals  have  been  directed  toward  food  getting  and  food 
digestion  and  protection  against  enemies  who  would  make  food 
of  them,  functions  involving  highly  developed  muscular  sys- 
tems, closely  correlated  nervous  responses  and  centralized 
nervous  systems,  and  complicated  organs  for  the  digestion  of 
many  different  kinds  of  food.  Their  metabolism,  therefore, 
involves  active  destructive  processes  and  the  formation  of 
excessive  waste  matters,  for  the  disposal  of  which  complicated 
excretory  organs  have  been  evolved.  Higher  plants,  on  the 
other  hand,  are  stationary;  their  food  material  being  everywhere 
about  them,  locomotor  organs  are  not  developed  and  their 
nervous  response  is  limited  to  protoplasmic  irritability;  waste 
matters  are  relatively  unimportant  and  easily  disposed  of,  re- 
quiring no  complicated  excretory  organs.  Their  plan  of  de- 
velopment, like  that  of  animals,  has  been  essentially  in  the 
service  of  nutrition.  Great  trunks  and  branches  have  been 
evolved,  apparently  in  response  to  the  need  of  presenting  maxi- 
mum chlorophyll-bearing  leafy  surfaces  to  the  air  and  light, 
while  great  subterranean  roots  absorb  water  and  salts  from  the 
earth.  Strong  frameworks  of  lifeless  wood,  giving  resistance  to 
winds  and  weather,  have  been  evolved  for  the  support  of  the 


106  PLANTS,  THE  FOOD  OF  ANIMALS 

heavy  aerial  structures,  while  complicated  canal  systems  for  the 
transportation  of  salts  and  foods  in  solution  penetrate  the  entire 
plant  organism  from  the  tiniest  rootlet  to  the  tips  of  the  highest 
leaves.  The  reproductive  organs,  finally,  are  equally  well 
developed  in  plants  and  animals,  the  maintenance  of  species 
being  a  universal  biological  need. 

Plants,  like  animals,  are  divided  into  two  great  groups,  proto- 
phyta  and  metaphyta,  although  these  designations  are  not 
often  used  in  classification.  The  metaphyta  bear  the  same 
relation  to  the  protophyta  that  the  metazoa  bear  to  the  pro- 
tozoa, viz.,  many-celled  as  contrasted  with  single-celled  organ- 
isms. For  our  purposes  a  glance  at  each  type  will  suffice  to 
give  a  clear  understanding  of  the  essential  relationships  of 
animals  and  plants,  and  show  that,  except  for  the  functions  of 
nutrition,  the  fundamental  biological  principles  in  animals 
and  plants  are  the  same. 

B.  PLEUROCOCCUS    PLUVIATILIS  AND    SPHAERELLA   LACUSTRIS 

These  two  organisms  are  good  types  of  the  unicellular  plants, 
the  former  existing  as  quiescent  non-motile  cells,  the  latter 
having  two  phases,  one  motile,  the  other  not.  As  unicellular 
forms  they  are  allied  to  a  large  number  of  low  types  of  plants 
included  in  the  group  known  as  Algae,  in  which  some  forms 
are  included  which  cannot  be  accurately  determined  as  plants 
or  as  animals.  Some  of  these,  like  the  Peridiniales  or  Dino- 
flagellata  and  the  Volvocidae,  are  included  by  botanists  as 
plants,  by  zoologists  as  animals. 

Pleurococcus  is  widely  distributed  in  damp  places,  where 
it  exists  as  a  green  covering  to  stones,  tree  trunks,  ground,  etc. 
Each  cell  is  composed  of  protoplasm  differentiated  into  cyto- 
plasm and  nucleus,  and  contains  minute  grains  of  starch. 
Green  coloring  matter,  chlorophyll,  is  uniformly  distributed 
throughout  the  cell,  which,  finally,  is  covered  by  a  transparent 
coating  of  cellulose,  a  characteristic  plant  product  similar  in 
chemical  composition  to  starch  but  with  a  different  arrange- 
ment of  molecules.  Reproduction  occurs  by  simple  division, 
the  daughter  cells  separating  when  formed,  or  remaining  to- 


UNICELLULAR  PLANTS 


107 


FIG.  40. — Pleurococcus,  from  the  bark  of  an  elm  tree  in  active  vegetation.  A , 
Dried  cells;  B,  division  within  a  cyst;  C,  cyst  contents  divided  into  four  cells; 
D,  motile  form  of  Prbtococcus  (?).  (From  Sedgwick  and  Wilson.) 


108 


PLANTS,  THE  FOOD  OF  ANIMALS 


gether  in  groups  of  two,  three  or  more  cells,  sometimes  eight  or 
nine  forming  a  loosely  arranged  colony.     The  union  is  only 


FIG.  41. — Sphaerella  lacustris.  Various  stages  in  vegetative  life  and  spore  for- 
mation. A,  a  typical  red-colored,  resting  cell;  B,  a  red  individual  with  two 
flagella  and  a  yellow-green  border  of  chlorophyll;  C,  an  older  individual  with 
more  chlorophyll  and  a  widely-distended  cell- wall;  D,  a  mature  green  individual 
with  red  eye-spot;  £,  individual  that  has  lost  its  flagella  and  developed  a  thick 
cell- wall;  F,  16  microzooids  formed  in  the  mother-cell;  G,  mature  megazooid; 
H,  7,  megazooid  formation.  (From  Hazen.) 

temporary,  however,  for  ultimately  the  cells  separate  and  live 
as  independent  units  (Fig.  40). 


UNICELLULAR  PLANTS  109 

Sphaerella  lacustris  in  its  quiescent  phase  is  similar  to  Pleuro- 
coccus,  save  for  the  presence  of  what  are  termed  chloroplastids, 
products  of  the  cell,  which  are  specialized  for  the  purpose 
of  manufacturing  chlorophyll,  now  confined  to  these  bodies. 
At  times  the  green  color  is  replaced  by  a  distinct  red  haema- 
tochrome,  which  is  only  a  masked  form  of  chlorophyll  and  is 
known  to  be  a  condition  brought  about  by  lack  of  nitrogen. 
The  flagellated  phase  of  Sphaerella  is  quite  different  in  appear- 
ance from  the  resting  form.  The  chief  structural  difference 
is  the  presence  of  two  definite  flagella,  originating  from  basal 
granules  in  the  cell  protoplasm  and  extending  through  the 
coating  of  cellulose  to  the  outside,  where  their  undulations 
in  the  water  lead  to  energetic  and  jerky  movements  of  the 
entire  organism.  A  red  so-called  "  eye-spot "  is  also  present, 
and  represents  a  more  sensitive  bit  of  protoplasm,  especially 
in  respect  to  light  (Fig.  41). 

Like  Pleurococcus,  Sphaerella  reproduces  by  simple  division, 
but  the  cells  do  not  form  colonies  or  remain  connected  after 
division.  At  times,  furthermore,  the  protoplasm  of  the  cell, 
protected  by  the  firm  cell  membrane,  divides  repeatedly  until 
from  thirty-two  to  sixty-four  minute  cells  are  formed.  These 
ultimately  break  out  of  the  cyst  and  swim  about  by  means  of 
two  flagella.  Two  of  these  small  products,  upon  meeting, 
fuse,  lose  their  flagella,  and  settle  down  as  a  resting  cell.  This 
process  of  conjugation  is  a  primitive  type  of  sexual  repro- 
duction, and  the  minute  cells  may  be  called  gametes. 

The  chief  interest  of  Pleurococcus  and  Sphaerella  lies  in 
their  physiological  activites.  Surrounded  by  a  membrane  of 
cellulose  and  an  even  more  resistant  cell  membrane,  solid 
matters  cannot  enter  the  cell.  Salts,  however,  dissolved  in 
water  can  be  absorbed  by  osmosis  through  the  body  wall,  and 
gases  can  diffuse  through  the  cellulose.  In  this  way  the  plant 
cells  take  in  CO2,  water,  salts  of  various  kinds,  and  give  out  CO2, 
free  oxygen,  and  waste  matters,  none  of  which  has  much 
chemical  energy.  The  carbon  dioxide  and  water  are  broken 
down  into  their  constituent  parts  through  the  energy  of  sun- 
light acting  through  the  chlorophyll,  and  the  elements  thus 


110  PLANTS,  THE  FOOD  OF  ANIMALS 

freed  are  ultimately  recombined  into  sugars  and  starch,  from 
which,  by  a  series  of  changes  which  can  best  be  described,  in 
connection  with  a  higher  type-  of  plant,  they  are  finally  made 
into  protoplasm;  this,  when  life  is  extinct,  becomes  protein,  the 
main  food  of  animals. 

The  plant  forms  serving  as  food  for  the  higher  animals  are  far 
more  complicated  than  Pleurococcus  and  Sphaerella,  and  just 
as  the  higher  animals  become  progressively  differentiated  with 
complicated  organs  for  the  performance  of  the  functions  of  food 
getting,  digestion,  assimilation,  excretion,  nervous  response 
and  reproduction,  so  do  the  higher  plants  become  progress- 
ively differentiated  with  complicated  organs  for  the  perform- 
ance of  their  metabolic  and  reproductive  functions. 

To  trace  the  food  of  animals,  therefore,  it  is  necessary  to  ex- 
amine the  structure  and  functions  of  the  higher  plants,  a  good 
example  of  which  is  the  common  fern  or  brake,  Pteridium  aquili- 
num, formerly  called  Pteris  aquilina. 

C.  PTERIDIUM  AQUILINUM 

The  common  brake  or  fern,  Pteridium  aquilinum,  is  widely  dis- 
tributed upon  the  earth's  surface,  growing  in  damp  or  shady 
places  and  resisting  various  kinds  of  unfavorable  conditions  of 
the  environment.  At  one  time  in  the  earth's  history — the  age 
of  Pteridophytes — ferns  formed  the  chief  type  of  vegetation, 
and  some  of  them  grew  to  an  enormous  size  (up  to  sixty  feet), 
while  even  today  some  ferns  are  tree-like  in  size  and  mode  of 
growth  (tree-ferns).  Others,  like  the  maiden  hair,  are  extremely 
delicate,  growing  only  in  the  most  favorable  localities. 

For  purposes  of  description,  Pteridium  may  be  regarded  as 
composed  of  two  distinct  parts;  the  one,  aerial  or  above  ground, 
is  termed  the  frond  or  leaf  and  consists  of  the  chlorophyll-bear- 
ing parts  and  their  supporting  and  nutritive  organs;  the  other, 
underground,  is  termed  the  rhizome  and  consists  of  a  stem 

FIG.  42. — Pteridium  aquilinum.  The  underground  stem  or  rhizome  (rh.),  one 
frond  (I1)  of  the  present  year  in  full  leaf,  the  other  (I2)  of  the  past  year;  ab,  apical 
bud  bearing  apical  cell  at  the  extremity  of  a  branch  bearing  stumps  of  leaves  of 
previous  seasons;  I1,  mature  active  leaf;  I2,  dead  leaf  of  the  preceding  year;  l.m., 
lamina  of  leaf;  p,  pinna;  x}  younger  pinna  shown  enlarged  at  B.  (After  Sedg- 
wick  and  Wilson.) 


THE  COMMON  BRAKE 


111 


Lm> 


FIG.  42. 


112 


PLANTS,  THE  FOOD  OF  ANIMALS 


with  roots  and  rhizoids  stretching  out  in  all  directions  for  the 
absorption  of  water  and  salts.  The  fronds  grow  up  at  intervals 
from  the  underground  stem,  a  single,  vigorous  rhizome  often 
bearing  several  fronds  (Fig.  42). 

The  Rhizome. — The  main  stem  of  the  underground  .part  may 
be  a  simple  root-like  structure  of  practically  uniform  size  and 
several  feet  in  length,  or  it  may  be  branched,  with  numerous 
lateral  rhizomes  penetrating  the  earth  in  different  directions. 
The  main  rhizome  and  its  branches  lie  a  few  inches  below  the 
surface  and  always  parallel  with  that  surface,  while  roots  grow 
out  from  it  into  the  earth  below.  In  addition  to  the  roots  there 


li 


FIG.  43. — Branch  of  a  rhizome  of  Pteris  showing  the  apical  bud  (a.6.)  stumps  of 
numerous  leaves  (I1,  I2,  etc.),  and  a  part  of  the  main  rhizome  (rh.)\  r,  roots. 
(From  Sedgwick  and  Wilson.) 

are  numerous  filamentous  outgrowths,  termed  hairs  or  rhizoids, 
which  differ  in  finer  structure  from  the  roots.  One  end  of  the 
rhizome  consists  of  soft  white  tissue  quite  different  in  appear- 
ance from  the  black  surface.  These  are  the  growing  points  of 
the  rhizome  and  branches,  all  growth  of  the  underground  trunk 
or  stem  being  in  a  linear  direction,  and  new  cells  are  added  by 
growth  of  the  terminal  cell,  termed  the  apical  cell  (Fig.  43). 

HISTOLOGY 

A  cross  section  of  a  rhizome  (Fig.  44)  shows  that  it  is  not 
quite  circular  in  outline,  the  transverse  axis  being  somewhat 


HISTOLOGY  OF  THE  FERN  113 

longer  than  the  vertical;  the  two  sides,  furthermore,  show  some- 
what flattened  ledge-like  surfaces  which  are  turned  upward, 
a  differentiation  offering  more  resistance  to  up-rooting  than 
would  be  the  case  if  the  sides  were  smoothly  rounded.  The 
cross  section  also  shows  various  cellular  differentiations.  On 
the  outside  is  a  layer  of  cells  termed  the  cortex,  consisting  of  an 
outer  black  and  lifeless  layer  to  which  the  name  epidermis  is 
given,  while  below  this  is  a  layer  of  hardened,  almost  wood-like, 


FIG.  44. — Transverse  section  of  a  vascular  bundle  surrounded  by  funda- 
mental tissue.  The  conducting  system  of  the  plant.  (From  Sedgwick  and 
Wilson.) 

cells  forming  the  bulk  of  the  cortex.  These  cortex  cells  are 
hardened  by  the  lignification  of  the  protoplasmic  structures, 
this  lignin  forming  the  chief  element  in  the  composition  of  the 
still  more  hardened  wood  tissues  within  the  rhizome.  Within 
the  cortex  is  a  mass  of  living  parenchyma  cells  with  soft  and  un- 
differentiated  protoplasm,  well  supplied  with  starch,  and  form- 
ing the  bulk  of  the  mass  of  the  rhizome.  Scattered  among  these 
parenchyma  cells  are  two  large  masses  and  other  smaller  patches 
of  dark  brown  material  formed  by  the  lignification  of  the  funda- 


114 


PLANTS,  THE  FOOD  OF  ANIMALS 


mental  cells,  all  firmly  attached,  and  forming  a  tough  and  re- 
sisting framework  or  skeleton,  giving  strength  and  rigidity 
to  the  whole.  These  woody  masses  are  together  called  the 
stereome.  Finally  there  are  smaller  and  more  or  less  circular 
patches  of  cells  with  thick  cellulose  walls,  which  form  the  most 
important  organs  of  the  rhizome,  the  aggregates  being  termed 
vascular  bundles. 

If  we  examine  a  vertical  or  horizontal  section  of  the  rhizome 
(Fig.  45)  we  find  that  these  internal  masses  of  stereome  and  the 
vascular  bundles  are  composed  of  elongate  woody  and  cellular 
structures,  firmly  attached  end  to  end  so  that  continuous  sup- 


FIG.  45. — Longitudinal  section  of  a  vascular  bundle  showing  the  conducting 
system  of  the  plant  in  lengthwise  section.     (From  Sedgwick  and  Wilson.) 

porting  structures  and  tubes  traverse  the  rhizome  from  end  to 
end,  the  former  serving  as  an  internal  skeleton,  the  latter  as 
conducting  and  feeding  organs. 

In  the  immediate  vicinity  of  the  growing  tip  of  the  rhizome 
the  cells,  with  the  exception  of  the  apical  cell,  are  practically  all 
alike  and  of  the  fundamental  parenchyma  type  forming  the 
primary  meristems,  but  they  become  differentiated  at  a  short 
distance  from  the  apical  cell  to  form  cells  of  varying  structure 
and  function,  some  becoming  vascular  cells,  while  others  are 
transformed  into  lifeless  stereome. 

These  various  groups  of  cells,  tubes,  supporting  structures, 
etc.,  are  not  only  continuous  throughout  the  main  trunk  of  the 
rhizome  but  are  also  continuous,  through  branches,  in  every 


HISTOLOGY  OF  THE  FERN 


115 


root  and  every  leaf  stalk,  the  root  hairs  alone  being  free  from 
them.  The  canal  system  leading  into  the  stalks  of  the  fronds 
are  the  most  important,  forming,  as  they  do,  the  only  means 


FIG.  46. — Epidermis  from  the  under  side  of  a  leaf,  showing  the  wavy  outlines 
of  the  epidermal  cells,  the  veins  (v)  covered  by  thickened  epidermal  cells,  and 
guard  cells  (s.t.)  enclosing  the  stomata.  (From  Sedgwick  and  Wilson.) 

of  communication  between  the  aerial  frond  and  the  underground 
parts  of  the  fern. 

The  Leaf  or  Frond. — The  aerial  part  of  Pteridium  consists  of  a 
main  branch  or  stem  arising  as  an  outgrowth  from  the  rhizome. 


116 


PLANTS,  THE  FOOD  OF  ANIMALS 


Its  branches,  termed  pinnae,  in  turn  give  rise  to  the  flattened 
chlorophyll-bearing  structures  analogous  to  leaves  of  higher 
plants,  but  here  termed  pinnules.  (In  the  comparative  mor- 
phology of  plants  analogous  parts  do  not  always  bear  the  same 
names,  thus  the  leaf  of  the  fern  is  the  entire  aerial  part  of  the 
plant  while  the  rhizome  corresponds  to  the  trunk  of  a  tree. 
The  trunk  is  underground,  the  leaves  alone  being  exposed.) 


ep* 


FIG.  47. — Cross  section  of  a  portion  of  a  leaf  showing  the  epidermis  (ep.), 
the  palisade  mesophyll  (above)  and  the  spongy  mesophyll  (below).  Sections 
through  the  guard  cells  and  stomata  (st.)  show  the  openings  into  the  inter-cellu- 
lar spaces  (i.s.).  (From  Sedgwick  and  Wilson.) 

The  stalk  of  the  frond  is  somewhat  thickened  at  the  point  where 
it  enters  the  ground,  thus  giving  greater  resistance  to  wind,  etc. 
It  is  generally  supposed  that  the  frond  is  only  a  much  thinned 
or  flattened  outgrowth  of  the  stipe  or  stem.  It  is  composed 
of  the  same  series  of  cells  as  those  found  in  the  rhizome,  but 
some  of  them  have  undergone  modifications.  The  epidermis 
cells  of  the  rhizome  are  lifeless,  but  here  they  are  living  and 
elaborated  into  flattened  epidermal  cells  with  curious  wavy  out- 


HISTOLOGY  OF  THE  FERN  117 

lines,  which  form  the  outer  covering  for  both  the  upper  and  the 
under  sides  of  the  leaf.  The  epidermis  cells*  are  colorless  for 
the  most  part,  but  here  and  there  among  them  bright  green 
chlorophyll-bearing  cells  may  be  seen  in  pairs  (Fig.  46,  s.t.). 
These  cells  are  bean-shaped  and  surround  a  minute  pore  termed 
the  stoma^  which  connects  the  inner  air  spaces  of  the  leaf  and  the 
surrounding  atmosphere.  They  further  have  the  function  of 
swelling  or  of  decreasing  in  size  with  the  humidity  of  the  air,  and 
thus  regulate  the  openings  of  the  stomata;  for  this  reason,  the 
term  guard  cells  is  usually  given  them.  On  the  upper  surface  of 
the  leaf  the  epidermal  cells  are  continuous,  and  guard  cells  are 
absent  but  they  are  widely  distributed  on  the  lower  surface. 

The  fundamental  parenchyma  of  the  rhizome  is  continued 
into  the  leaf,  but  a  new  function  is  there  undertaken  by  the 
generalized  cells.  They  become  large  cuboidal  cells,  closely 
packed  together  on  the  upper  side  forming  a  palisade  mesophyll 
layer,  while  on  the  under  side  they  are  loosely  arranged  with 
relatively  great  gaps  or  chambers,  forming  the  spongy  meso- 
phyll. The  chambers  are  in  communication  with  the  outer 
air  by  means  of  the  stomata.  The  term  mesophyll,  or  some- 
times chlorenchyma,  is  applied  to  these  cells  because  of  the 
universal  presence  of  chloroplastids  colored  green  by  chloro- 
phyll (Fig.  47). 

The  vascular  bundles  break  up  in  the  leaves  into  a  series 
of  fine  tubes  which  are  differentiated  for  collecting  food  sub- 
stances, and  for  conducting  fluids,  while  the  stereome  is  re- 
duced to  a  minimum. 

Throughout  the  protoplasm  of  the  mesophyll  cells  are  prod- 
ucts of  cellular  activity  in  the  form  of  minute  spherical  or 
tabloid  granules,  termed  chloroplastids.  They  are  only  a 
modified  form  of  protoplasm  and  have  the  power  to  reproduce 
themselves  by  division;  hence  they  are  living  elements  of 
plant  protoplasm  and  are  often  colorless,  especially  in  the 
dark.  In  the  light  these  chloroplastids  have  the  power  of 
forming  an  oily  fluid  substance  of  green  color,  the  chlorophyll, 
which  disappears  after  some  time  in  the  dark,  but  can  be  re- 
formed in  the  light.  Its  chemical  composition  is  very  complex 
and  is  of  the  nature  of  protein,  its  formula,  as  given  by  Willstat- 


118  PLANTS,  THE  FOOD  OF  ANIMALS 


ter,  being  something  like 
H3g).  If  white  light  be  passed  through  a  prism  it  is  broken  up 
into  the  colors  of  the  spectrum;  if  passed  through  a  chlorophyll 
solution  it  shows  absorption  bands  in  the  red,  yellow,  green, 
blue  and  violet,  thus  indicating  the  absorption  by  the  chloro- 
phyll of  the  sun's  rays  richest  in  actinic  energy.  This  energy 
is  utilized  by  the  plant  in  reducing  C02  and  H2O,  a  first  step  in 
the  manufacture  of  the  plant's  food.  Chlorophyll,  finally,  is 
easily  split  up  into  cyanophyll  with  a  blue-green  color,  and 
xanthophyll  with  a  yellow  color,  while,  in  the  presence  of  acid, 
the  Mg  is  replaced  by  hydrogen,  giving  a  magnesium-free 
yellow  derivative,  termed  phaeophytin. 

GENERAL   PHYSIOLOGY 

Food  materials  for  the  fern  include  a  large  variety  of  simple 
elementary  compounds  found  everywhere  in  the  soil  and  air. 
From  the  soil,  salts  of  different  kinds  are  absorbed  by  the  roots, 
and  pass  by  means  of  the  vascular  bundles  to  all  parts  of  the 
plant;  water,  holding  salts  in  solution,  is  also  taken  in  by 
these  organs,  and  passes  by  osmosis  and  root  pressure,  aided  by 
evaporation  in  the  leaves,  to  the  highest  parts  of  the  aerial 
plant.  From  the  air,  carbon  dioxide  and  oxygen  are  taken  in, 
and  by  aid  of  the  energy  taken  from  sunlight,  the  carbon  is 
dragged  away  from  the  oxygen,  and  the  hydrogen  likewise 
from  oxygen,  leaving  these  elements  ready  to  recombine  pre- 
paratory to  the  formation  of  sugars  and  starch.  For  this  proc- 
ess it  was  formerly  supposed  that  a  number  of  molecules  of 
carbon  were  united  with  twice  as  many  molecules  of  water,  but 
now  it  is  considered  more  probable  that  the  base  of  the  opera- 
tion is  the  hydroxyl  OH.  The  reaction  is  usually  expressed  in 
the  following  manner,  although  the  equation  does  not  represent 
all  of  the  actions  taking  place:  n5H2O  +  n6C02  =  nC6Hi005  or 
starch  +  n6O2.  ,  It  is  more  probable  that  the  reaction  is 
brought  about  through  the  formation  of  intermediate  products, 
thus:  C02  +  H2O  =  CH2O  +  O2.  Or  possibly,  CO2  +  3H2O 
=  CH20  +  2H202,  the  latter,  hydrogen  peroxide,  breaking 
down  into  H2O  and  O2.  CH2O  is  a  poison,  formaldehyde,  and 


PLANT  PHYSIOLOGY  119 

must  be  condensed  immediately  upon  its  formation,  presumably 
changing  into  a  simple  hexose  sugar  by  addition  and  rearrange- 
ment of  its  molecules,  thus  n6CH20  =  nC6Hi2O6  or  glucose  from 
which  starch  is  formed  by  the  loss  of  water,  thus  nCeH^Oe  — 
nH2O  =  nCeHioO5  or  starch.  This  starch  is  stored  temporarily 
in  the  leaves,  or  it  is  gathered  up  as  glucose,  which  is  soluble, 
by  the  collecting  tubes  and  carried  through  the  vascular  bundles 
to  the  rhizome  where,  in  the  parenchyma  cells,  it  is  permanently 
stored  as  starch  to  be  used  as  needed  by  the  plant.  At  night, 
in  this  way  the  starch  is  removed  from  the  leaves,  but  with  the 
advent  of  daylight  the  manufacturing  process  begins  again. 
This  process  of  starch  manufacture  by  photosynthesis  is  the 
essential  difference  between  animals  and  plants,  and  the  plant 
has  this  power  by  virtue  of  chlorophyll. 

Recent  investigations  in  the  chemistry  of  chlorophyll  indi- 
cate that  the  reactions  taking  place  in  the  formation  of  hexose 
sugars  are  far  more  complex  than  those  outlined  in  the  preced- 
ing paragraph.  Thus  pure,  extracted  chlorophyll,  used  as  a 
sol  with  water  as  the  dispersion  medium,  in  a  closed  vessel  con- 
taining only  carbon  dioxide,  and  exposed  to  light,  will  not  pro- 
duce formaldehyde.  The  carbon  dioxide  and  water  form  car- 
bonic acid,  and  this  causes  the  displacement  of  the  chlorophyll 
magnesium,  thus  producing  yellow  phaeophytin  (Willstatter) . 
On  the  other  hand,  similarly  prepared  chlorophyll  in  a  closed 
vessel  containing  oxygen  and  exposed  to  the  light,  will  lose  its 
color  entirely,  while  formaldehyde  is  formed  in  variable  quan- 
tities depending  on  the  length  of  time  of  exposure,  while  the 
acidity  of  the  system  continually  increases.  This  production 
of  formaldehyde  may  be  interpreted  as  due  to  oxidation  and 
destruction  of  the  chlorophyll  molecule  through  splitting  off  and 
reduction  of  the  alcohol  ester  (phytol)  contained  in  that  mole- 
cule, while  the  increasing  acidity  may  be  due  to  the  further 
oxidation  of  the  formaldehyde  to  formic  acid  (Jorgensen  and 
Kidd). 

If  formaldehyde  production  in  the  living  plant  is  a  necessary 
step  in  the  formation  of  plant  sugars,  and  if  it  is  formed  in  the 
manner  outlined  by  Jorgensen  and  Kidd,  then  the  process  of 


120  PLANTS,  THE  FOOD  OF  ANIMALS 

sugar  formation  is  not  a  direct  result  of  constructive  metabolism, 
but  rather  a  direct  effect  of  destructive  metabolism  through 
oxidation.  In  the  test  tube  the  chlorophyll  is  destroyed  by 
this  chemical  change,  but  chlorophyll  in  the  test  tube  is  quite 
another  matter  from  chlorophyll  in  the  living  leaf  where,  if  such 
reactions  are  necessary  in  the  formation  of  sugar,  they  are 
balanced  by  the  synthesizing  activity  of  the  chromogen  com- 
plex (MgN4C32H3oO)  in  the  presence  of  C02  and  H2O  and  with 
the  energy  of  sunlight.  The  chromogen  complex  would  thus 
play  the  part  of  a  synthesizing  enzyme  activated  in  both  con- 
structive and  destructive  phases  by  the  energy  of  light.  Pres- 
ent knowledge,  however,  is  very  incomplete  in  regard  to  photo- 
chemistry of  the  chloroplast,  and  such  deductions,  while  allur- 
ing, must  be  regarded  as  purely  hypothetical,  although  no 
more  hypothetical  than  the  long-accepted  view  of  the  direct 
union  of  CO2  and  H^O  in  the  formation  of  hexose  sugars  by 
photosynthesis. 

From  this  point  on  in  nutrition,  animals  and  plants  alike  have 
the  power  to  manufacture  proteins.  In  the  plant,  the  process 
can  be  followed  more  easily  than  in  animals;  some  of  the  simpler 
compounds  like  asparagin  consist  of  nitrogen  added  to  the 
carbohydrate  (C^jHs^O^,  and  from  this  relatively  simple 
amide,  protein  may  be  formed  by  the  addition  of  the  essential 
elements.  The  formation  of  these  substances  is  obscure,  the 
action,  presumably,  being  brought  about  through  the  agency  of 
synthesizing  enzymes.  In  animals,  the  protein  materials 
taken  as  food  provide  the  necessary  elements  for  this  synthesis, 
but  in  plants  the  proteins  must  be  built  up  step  by  step.  Plants 
thus  are  essentially  constructive  while  animals  are  destructive. 

In  the  manufacture  of  starch  more  oxygen  is  liberated  from 
combination  than  can  be  used,  and  this  diffuses  through  the 
leaves  and  into  the  air,  while  carbon  dioxide  is  taken  in. 
Plants  and  animals,  therefore,  would  seem  to  be  well  adapted 
for  mutual  existence  side  by  side.  But  the  plant  does  more  or  less 
work  and  utilizes  its  substance  in  providing  the  energy  neces- 
sary for  this  work,  while  waste  matters,  in  the  form  of  C02  and 
water  and  nitrogenous  substances,  are  formed.  As  in  animals, 


CYCLE  OF  MATTER  AND  ENERGY 


121 


the  CO2  is  given  off  into  the  air  while  oxygen  is  taken  into  the 
plant  from  the  air,  but  this  fundamental  process  of  respira- 
tion is  masked  by  the  more  active  processes  going  on  under 
the  action  of  chlorophyll,  so  that,  in  sunlight,  the  oxygen  needed 
is  provided  by  the  residue  of  oxygen  after  starch  is  formed, 
the  remainder  being  given  off,  while  the  waste  matter  CC>2  is 
reduced  and  ultimately  manufactured  into  starch.  The  essen- 
tials of  respiration,  therefore,  go  on  all  the  time,  but  C02  is  actu- 


ERlA  .  ACETIC  ACID 
*  BACTERIA  -CO  . 

I  I 


5ALT5(Ca,  Ha,  P,  Fe.TTi<j,  5.  nttj.ffcj          CCfe 
NITRATES  .  NITWTE5 


.       CO       NITf?ATE5 

'        PHOSPHATES 

OTHER  SALTS 


EARTH -"WATER 


PIG.  48. — Diagram  illustrating  the  cycle  of  living  matter  and  energy  in  animals, 
plants,  yeast  and  bacteria. 

ally  given  off  by  the  plant  only  in  the  absence  of  sunlight.  The 
oxygen  given  off  cannot  be  considered  a  waste  matter  or  prod- 
uct, since  it  has  not  been  a  part  of  the  plant  organism  but 
is  only  a  by-product  of  chlorophyll  activity.  Nitrogenous 
waste  may  be  disposed  of,  when  present  in  small  quantities,  by 
being  stored  in  the  leaves,  and  finally  cast  off  by  the  annual 
shedding  of  leaves  and  their  contents. 

Cycle  of  Matter  and  Energy. — The  autotrophic  nutrition  of  the 
fern,  and  of  plants  generally,  is  the  starting  point  for  the  wonder- 


122  PLANTS,  THE  FOOD  OF  ANIMALS 

ful  cycle  of  matter  and  energy  in  nature  whereby  living  things 
are  all  interrelated  and  balanced.  Plants  manufacture  pro- 
teins which  are  built  up  into  animal  protoplasm.  Plants  also 
produce  glucose  which,  acted  upon  by  yeast,  is  transformed  into 
alcohol  and  CO2.  The  alcohol  is  acted  upon  by  bacteria  and 
changed  to  acetic  acid  and  water,  other  bacteria  act  upon  this 
acetic  acid  and  change  it  to  CO2  and  water.  Plants  and  ani- 
mals die,  their  protoplasm,  as  protein,  is  acted  upon  by  bacteria 
and  broken  down  into  free  ammonia,  nitrites,  nitrates,  sulphates, 
phosphates  and  other  salts,  all  of  which  are  returned  to  the  earth 
to  be  taken  up  by  the  roots  of  plants  and  built  again  into  plant 
protoplasm.  Animals,  and  to  a  less  extent,  plants,  produce 
nitrogenous  waste  as  a  product  of  metabolism.  This  is  acted 
upon  by  bacteria  and  turned  into  NHs  and  CO2  and  water. 
In  this  way  there  is  a  continual  cycle  of  simple  salts  and  gases 
converted  into  starches,  sugars,  plant  and  animal  protein  with 
high  potential  energy  which  is  ultimately  transformed  into 
energy  of  heat,  light,  electricity  and  movement,  giving  the 
infinite  variety  of  vital  manifestations.  This  protein,  through 
oxidizing  agents  and  nitrifying  agents,  is  finally  brought  again  to 
the  state  of  elementary  compounds.  All  may  be  shown  in 
a  simple  diagram  (Fig.  48) . 

REPRODUCTION   OF   THE   FERN 

The  rhizome  of  the  fern  may  give  rise  now  and  then  to  branch 
rhizomes  which  start  up  independent  plant  growths,  and  thus 
bring  about  a  form  of  reproduction  somewhat  analogous  to 
budding  in  Hydra.  This,  however,  is  only  an  exceptional 
method  of  reproduction  and  does  not  amount  to  much  in  the 
distribution  of  the  fern.  The  chief  methods  of  reproduction 
do  not  involve  the  rhizome  at  all,  but  take  place  as  a  result  of 
activity  of  the  frond  cells.  As  in  hydroids,  reproduction  here 
involves  an  alternation  of  generations,  sexual  and  asexual 
generations  following  each  other  in  regular  succession. 

The  Asexual  Generation  (Sporophyte). — The  ordinary  fern 
plant  is  the  asexual  generation,  i.e.,  it  does  not  form  the  sex  cells 


REPRODUCTION  OF  THE  FERN 


123 


but,  like  the  hydroid,  it  gives  rise  without  fertilization  to  an 
organism  different  from  itself.  These  dissimilar  organisms  are 
formed  from  spores  which  develop  on  the  under  surfaces  of  the 


FIG.  49. — Development  of  a  fern  (Aspidium)  sporangium,  a,  The  young  spor- 
angium-forming cell  just  divided  from  its  parent  epidermis  cell;  b,  c,  d,  e,  f, 
different  aspects  of  the  dividing  cells  of  the  spore  capsule;  g,  origin  of  the  tapetal 
cells  and  formation  of  the  spore-producing  cell  or  archesporium  (ar);  h,  increase 
of  the  tapetal  cells  (i)  and  formation  of  the  spore  mother  cells;  i,  j,  k,  further 
stages  in  the  development  of  the  sporangium;  an,  annulus;  pd,  pedicel.  (From 
Sedgwick  and  Wilson.) 


124  PLANTS,  THE  FOOD  OF  ANIMALS 

leaves.  In  nature,  sporulation  of  Pteridium  usually  occurs  in 
August,  and  in  allied  forms  sometime  during  the  summer 
months.  The  margins  of  the  mature  leaves,  when  ready  for 
spore  formation,  turn  under  and  form  elongated  pockets  which 
extend  throughout  the  length  of  the  pinnules.  This  inturned 
shelf  of  tissue  is  termed  the  false  indusium,  while  another  shelf 
of  tissue,  derived  from  the  epidermis  of  the  under  surface  and 
extending  out  to  the  false  indusium,  is  called  the  indusium,  the 
spore-bearing  organs  being  formed  in  the  chamber  enclosed 
by  the  true  and  false  indusia  and  the  under  surface  of  the 
pinnule. 

In  other  types  of  fern  the  spore  chambers  are  somewhat 
differently  constructed.  In  the  maiden-hair,  for  example,  the 
entire  edge  of  the  pinna  is  not  turned  in,  but  three  or  more  spots 
on  the  edge  become  localized  spore-forming  centers,  each  cov- 
ered by  an  indusium.  In  the  Boston  fern  a  row  of  similar 
spots  on  each  side  of  the  median  line  on  the  under  surface 
are  spore-forming  centers;  each  spot,  termed  a  sorus,  is  covered 
by  an  indusium. 

The  spores  develop  in  peculiarly  shaped  spore-cases  called 
sporangia,  many  of  which  are  formed  in  a  sorus,  and  multitudes 
in  the  spore  chambers  of  Pteridium.  Each  sporangium  begins 
by  the  division  of  an  epidermal  cell  (Fig.  49,  a-h)  until  a  capsule 
is  formed,  with  a  ridge  (annulus)  of  specially  hardened  cells. 
Within  the  capsule  a  single  germ  cell,  the  archesporium,  di- 
vides six  consecutive  times,  forming  64  spores,  each  spore  being 
enclosed  in  a  firm  covering  (shell  or  epispore,  Fig.  49, 1).  When 
ripe,  the  sporangium  bursts  open  by  contraction  of  the  cells 
of  the  annulus,  and  the  spores  are  scattered  from  the  leaves  to 
the  ground. 

The  Sexual  Generation  (Gametophyte) . — After  some  months  on 
the  ground  the  spores  absorb  moisture,  the  epispore  bursts 
open,  and  the  spore  cell  or  endospore  begins  to  swell  and  to 

FIG.  50. — Germination  of  the  spore  and  formation  of  the  prothallium.  A, 
Young  plant  leaving  the  spore  case;  B,  similar  stage  after  one  cell  division  has 
occurred  (p,  protenema;  s,  spore  case;  r,  root).  Later  stages  in  formation  of  the 
young  prothallium  are  shown  on  the  left,  and  below  a  fully  developed  prothallium 
with  archegonia  and  antheridia.  In  the  notch  above  is  a  figure  (life  size)  of  the 
same  prothallium.  (From  Sedgwick  and  Wilson  after  Suminski.) 


REPRODUCTION  OF  THE  FERN 


125 


FIG.  50. 


126  PLANTS,  THE  FOOD  OF  ANIMALS 

divide,  forming  root-like  hairs  (rhizoids)  and  the  embryonic 
plant  termed  the  protonema  (Fig.  50).  The  end  cells  of  the 
protonema  develop  chlorophyll,  divide,  and  ultimately  form  a 
flattened  plate  of  cells  closely  applied  to  the  ground,  to  which  it 
is  anchored  by  the  rhizoids.  This  flat  plate  of  cells,  or  thallus, 
is  the  sexual  generation  of  the  fern  and  is  called  the  prothallium 
(Fig.  50).  It  is  entirely  unlike  the  fern  plant,  but  when  mature 
it  bears  the  sex  cells  which,  after  fertilization,  develop  into  the 
fern.  It  thus  resembles  the  medusa  of  a  hydroid,  an  organism 
quite  different  from  the  hydranth  from  which  it  came,  but  the 
sole  agent  in  the  formation  of  the  male  and  female  germ  cells 
which,  on  fertilization,  give  rise  to  the  hydroid. 

The  sex  cells  of  the  fern  are  formed  in  characteristic  organs 
on  the  under  side  of  the  prothallium.  The  oospheres  or  egg 
cells  are  developed  and  contained  in  peculiar  chimney-shaped 
structures  termed  archegonia  (Fig.  50);  while  the  male  cells 
are  formed  in  smaller  rounded  or  hemispherical  structures 
termed  antheridia  (Fig.  50).  The  two  types  of  structure  are 
each  formed  by  continued  division  of  an  epidermal  cell.  In 
the  archegonium  these  divisions  result  in  a  solid  column  of 
cells,  with  the  oosphere  embedded  at  the  base  of  the  column. 
The  central  cells  of  the  column  undergo  liquefaction,  thus  form- 
ing a  passage  filled  with  a  mucilaginous  liquid  from  the  apex 
of  the  archegonium  to  the  egg  cell.  The  antheridia  are  formed 
by  divisions  of  similar  epidermal  cells  which  develop  into  a  solid 
hemispherical  mound,  the  internal  cells  of  which  divide  re- 
peatedly; the  final  divisions  form  the  male  cells  or  anther  o- 
zoids,  each,  when  mature,  bearing  a  spiral  filament  covered 
with  cilia. 

The  antherozoids  usually  develop  first,  and  are  distributed  on 
the  ground  where  they  make  their  way  in  the  moisture  on  the 
under  side  of  the  prothallium  to  the  archegonia  of  the  same  or  of 
different  origin.  They  are  attracted  toward  the  chimney-like 
opening  of  the  archegonia;  one  or  more  penetrate  the  gelatin- 
ous passage  to  the  egg  cell,  and  one  antherozoid  unites  with  it. 
The  entire  process  of  fertilization  takes  place  within  the  tissues 
of  the  prothallium. 


REPRODUCTION  OF  THE  FERN 


127 


Development. — The  fertilized  egg  cell  or  oospore  begins  at  once 
to  divide,  first  into  two,  then  into  four  cells.  Of  these  first  four 
cells,  two  form  the  foot  or  attachment  organ  by  which  the  young 
embryo  retains  its  position  in  the  tissues  of  the  prothallium ; 
em. 


rh. 


FIG.  51. — Development  of  the  fern  embryo.  A,  Section  showing  the  closed 
neck  (»)  and  the  planes  of  division  of  the  embryo  into  four  cells  (em) ;  B  and  C, 
later  stages  of  the  embryo  showing  the  beginning  of  apical  growth  and  formation 
of  the  first  leaf  and  rhizome.  The  other  figures  represent  later  stages  in  develop- 
ment, ar,  old  archegonia;  /,  foot;  I,  leaf;  p,  prothallium;  r,  root;  rh,  rhizoids. 
(From  Sedgwick  and  Wilson,  after  Hofmeister  and  Sachs.) 

one  forms  the  rhizome,  and  one  the  first  frond  or  leaf  (Fig.  51). 
The  young  fern  thus  develops  while  anchored  to  the  sexual 
generation.  A  second  leaf  is  soon  started  from  the  basal  por- 
tion; the  first  leaf  unfolds  in  the  light  and  the  cells  become  filled 


128  PLANTS,  THE  FOOD  OF  ANIMALS 

with  chlorophyll;  the  rhizome  elongates  by  apical  growth 
through  the  ground,  until  the  young  fern  plant  is  fully  estab- 
lished and  is  ready  to  make  and  store  up  starch.  The  prothal- 
lium  gradually  shrivels  up  and  disappears. 


The  work  done  by  the  fern  is  duplicated  by  every  type  of 
plant  life  provided  with  chlorophyll.  Sugars,  starches,  proteins 
and  lifeless  woods  are  manufactured  and  stored  in  roots,  fruits, 
seeds,  and  trunks,  and  with  them  is  locked  up  the  potential  energy 
to  be  transformed  into  kinetic  energy  through  physiological 
and  physical  combustion  by  man  and  the  lower  animals.  Noth- 
ing is  wasted  in  the  life  cycle  of  matter  and  energy.  Sugars,  in 
addition  to  their  food  value  for  animals  and  plants,  in  the 
presence  of  yeast  are  transformed  into  alcohol  and  carbon  di- 
oxide, and  their  contained  energy  is  changed  into  heat,  energy 
of  yeast  protein  and  that  of  alcohol.  Alcohol,  in  the  presence  of 
bacteria,  is  turned  into  acetic  acid,  the  potential  energy  being 
converted  into  that  of  bacteria  protein  and  that  of  acetic  acid. 
The  latter  is  acted  upon  again  by  bacteria,  and  changed  into 
carbon  dioxide  and  water  in  which  the  contained  energy  is  nil, 
the  bacteria  protoplasm  again  storing  up  that  which  was  con- 
tained in  the  acetic  acid. 

The  proteins,  carbohydrates  and  fats,  derived  mainly  from 
plants,  in  the  last  analysis  are  the  main  foods  of  all  animals 
and  their  chief  sources  of  energy.  Both  plants  and  animals  re- 
lease the  stored  energy  in  the  form  of  heat,  light,  electricity  or 
movement,  and  in  metabolism  give  off  nitrogenous  waste,  car- 
bon dioxide,  and  water.  Of  these,  only  the  nitrogenous  sub- 
stances retain  some  energy  of  combination.  Under  the  action  of 
bacteria  this  small  store  is  transformed  into  energy  of  bac- 
terial protein,  while  free  ammonia,  carbon  dioxide  and  water 
are  returned  to  the  earth  and  the  air  (NH^CO  +  2H2O 
=  2NH3  +  CO2  +  H2O.  The  proteins  of  the  dead  bodies  of 
plants  and  animals,  after  furnishing  food  and  energy  for  scaven- 
gers of  many  kinds,  are  finally  attacked  by  the  army  of  nitrify- 
ing and  other  bacteria,  and  slowly  transformed  into  nitrites, 
nitrates,  sulphates,  phosphates  and  other  salts,  and  into  free 


SUMMARY  129 

ammonia,  carbon  dioxide  and  water.  Other  bacteria  are  able 
to  reduce  nitrates  and  nitrites,  and  thus  to  liberate  free  nitro- 
gen. These  are  relatively  few  in  number,  including,  however, 
the  well-known  species  Bacillus  coli,  B.  typhosus,  B.  fluorescens 
and  B.  pyocyaneus.  This  action  probably  follows  a  condition 
of  oxygen  hunger  on  the  part  of  the  bacteria  when  oxygen  is 
wrested  from  the  nitrogen,  leaving  the  latter  as  a  free  gas. 
Nitrogen,  thus  liberated,  cannot  be  utilized  by  the  higher  plants, 
and  were  it  not  for  the  activity  of  other  specially  adapted 
organisms  there  might  be  a  leak  in  the  current  of  matter  and 
energy.  It  is  still  a  disputed  question  whether  some  of  the 
lower  algae  and  some  of  the  common  molds  have  the  power  to 
"fix"  this  free  nitrogen  and  so  bring  it  back  into  the  cycle  of 
living  things.  This  function  is  performed,  however,  by  certain 
groups  of  bacteria,  some  of  which  are  able  to  bring  about  the 
oxidation  of  free  ammonia  (NHs)  to  nitrates,  while  still  other 
bacteria  can  fix  free  nitrogen  of  the  air  (Clostridium  pastorian- 
ium,  two  species  of  Azotobacter,  and  various  species  of  Bacillus). 
All  products  of  vital  activity  are  thus,  sooner  or  later,  returned 
to  the  earth  or  atmosphere  to  be  again  taken  up  by  the  green 
plants  and  animals,  and  drawn  once  again  through  the  living 
vortex  of  matter  and  energy. 

The  food  of  Hydra  thus  is  only  a  link  in  the  endless  chain 
of  matter-  and  energy-transformations  shown  in  the  diagram 
(p.  121).  Above  and  throughout  the  entire  marvelous  chain  of 
activities  is  the  silent  penetrating  agency  of  the  sun,  the  source 
of  all  energy. 


CHAPTER   VI 
ORGANS  AND  ORGAN  SYSTEMS 

i.  GENERAL 

IN  Hydra  fusca,  we  have  studied  an  animal  composed  only  of 
tissues,  and  a  form  representing  the  gastrula  stage  in  develop- 
ment of  all  higher  animals.  We  have  seen,  moreover,  that  some 
cells  of  Hydra,  especially  those  of  the  ectoderm,  are  differen- 
tiated for  special  functions,  particularly  those  for  protection  and 
nervous  response.  Such  cells,  however,  are  isolated  and  not 
combined  in  compact  tissues  or  special  organs.  In  higher  ani- 
mals, we  find  similar  cells  derived  from  the  ectoderm  of  the  gas- 
trula developing  into  complex  organs  of  the  skin  and  into  still 
more  complicated  organs  of  special  sense,  and  finally  into  mar- 
velously  intricate  organs  like  the  human  brain,  spinal  cord,  and 
sympathetic  nervous  system.  In  these  higher  animals  also,  cells 
from  the  endoderm  of  the  gastrula  form  all  of  the  organs  of 
the  usual  animal  metabolism.  All  of  the  usual  mesodermal 
structures,  like  bone,  cartilage,  muscles,  connective-tissues, 
mesenteries,  blood,  etc.,  are  derived  from  cells  originally  set 
apart  from  the  vegetative  cells  of  the  gastrula. 

An  organ  differentiated  for  the  special  performance  of  some 
vital  function  or  part  thereof  never  consists  solely  of  the  one 
tissue  mainly  responsible  for  that  function.  The  stomach,  for 
example,  in  which  the  first  stages  of  protein  digestion  occur,  does 
not  consist  merely  of  secreting  epithelial  cells  from  the  endo- 
derm, but  is  a  complicated  aggregate  of  connective  tissue,  mus- 
cle tissue  and  blood  vessels  from  the  mesoderm,  and  nerve  tis- 
sue from  the  ectoderm,  giving  support,  contractility  and  food  to 
the  functioning  epithelial  cells.  The  stomach,  furthermore,  per- 
forms only  a  part  of  the  function  of  food  digestion;  other  organs 

130 


HABITS  OF  EARTHWORMS  131 

are  associated  with  it  for  complete  digestion.  All  of  these,  like 
the  pharynx,  oesophagus,  liver,  pancreas,  and  intestine  and  rec- 
tal organs,  are  similarly  composed  of  different  tissues,  all  aiding 
in  the  one  function  of  preparing  food  for  the  body  or  of 
eliminating  indigestible  and  harmful  matters. 

Such  an  aggregate  of  organs  for  the  performance  of  a  pri- 
mary function  constitutes  an  organ  system.  The  aggregate  of 
stomach  and  accompanying  organs  is  termed  the  digestive 
or  alimentary  system.  All  of  the  nerve  organs  together  form 
the  nervous  system.  Similarly  we  find  in  all  higher  animals  an 
excretory  system;  a  supporting  and  muscular  system;  a 
respiratory  system;  a  blood  vascular  system  and  a  reproduc- 
tive system,  each  composed  of  organs  for  the  performance  of 
one  function  or  part  of  one  function.  All  of  the  vital  functions, 
like  digestion,  secretion,  respiration,  nervous  response,  repro- 
duction, etc.,  performed  by  the  single  cell  of  Amoeba  or  Par- 
amecium,  are  here  performed  by  complex  organ  systems. 

All  human  beings  are  familiar  in  a  general  way  with  the 
structures  and  functions  of  their  own  bodies,  and  they  realize, 
in  some  degree  at  least,  how  complicated  and  difficult  it  is  to 
understand  human  anatomy  and  human  physiology.  It  is  not 
only  desirable  but  essential,  therefore,  for  the  beginner  in  biol- 
ogy to  become  familiar  with  simple  types  of  organ  systems  and 
with  some  of  the  factors  which  have  led  to  the  differentiation  of 
such  relatively  simple  into  more  complex  systems,  before  under- 
taking a  study  of  the  highest  animal  or  deepest  biological  prob- 
lem in  detail.  It  is  to  this  end  that  the  present  and  the  follow- 
ing chapters  are  devoted. 

II.  STRUCTURES  AND  FUNCTIONS  OF  THE  EARTHWORM, 

LUMBRICUS    SP. 

The  earthworm  occupies  a  position  in  the  animal  scale  similar 
to  that  occupied  by  the  fern  in  the  plant  scale.  All  the  es- 
sential organ  systems  are  present,  but  the  general  organization, 
while  markedly  higher  than  that  of  Hydra,  is  relatively  simple 
when  compared  with  Crustacea,  insects  or  vertebrates. 


132  ORGANS  AND  ORGAN  SYSTEMS 

A.  OCCURRENCE,  HABITS  AND  MODE-  OF  LIFE  OF  EARTH- 
WORMS.— Earthworms  are  widely  distributed  over  the  earth,  and 
six  or  seven  different  species  are  known,  some  growing  to  giant 
size  (three  to  four  feet  long).  Closely  allied  forms  are  partly 
earth-dwelling,  partly  water-dwelling  forms,  and  some  live 
entirely  in  water,  while  one  type  (Dendrobaena)  burrows  in 
the  green  ice  of  glaciers  as  an  earthworm  burrows  in  the  earth. 

Earthworms  live  in  winding  burrows  formed  by  eating  their 
way  through  the  earth,  the  burrows  running  through  the  soil 
at  a  depth  of  from  five  to  six  inches  to  several  feet.  The  worms 
are  nocturnal  for  the  most  part,  coming  out  of  their  burrows  at 
night  to  forage.  During  the  day  they  lie  in  their  burrows, 
mouth  end  up  and  close  to  the  surface;  at  night  they  emerge,  but 
usually  remain  anchored  by  their  tails,  exploring  the  region  of 
their  burrows  throughout  the  area  covered  by  their  body 
radius.  Pebbles,  dirt,  leaves  and  other  small  objects  lying 
within  this  radius  are  swallowed  or  dragged  into  the  burrows, 
where  the  small  stones  are  used  to  line  the  walls  and  to  cover 
the  opening  in  the  daytime.  The  dirt  that  is  swallowed  with 
nutritious  matter,  such  as  leaves,  animal  remains,  etc.,  is  slowly 
passed  through  the  digestive  tract;  the  nutritious  parts  are 
digested  out,  while  the  residue,  consisting  mainly  of  dirt,  is 
voided  to  the  outside  through  an  opening  at  the  opposite  end, 
the  anus.  This  defecated  material  is  deposited  on  the  outside 
of  the  burrow,  where  as  small  mounds,  or  "castings"  or  ''faeces," 
triey  are  familiar  to  every  observer.  Darwin,  who  made  a 
special  study  of  earthworms,  has  shown  that  enormous  masses 
of  earth  pass  in  this  way  through  the  bodies  of  worms,  as  much 
as  eighteen  tons  per  acre  per  year  in  regions  where  earthworms 
abound.  He  has  also  shown  that  the  entire  surface  of  a  field 
with  great  rocks  upon  it,  in  the  course  of  several  years,  will 
be  buried  by  the  castings  of  worms,  while  walls  and  even  build- 
ings are  similarly  sunk  into  the  earth  in  the  course  of  time. 

As  a  creeping,  crawling,  and  burrowing  thing,  the  earthworm 
is  well  adapted  to  its  mode  of  life.  Its  long  flexible  body  makes 
it  particularly  adapted  to  its  burrowing  habits,  and  a  thought- 
ful student  will  puzzle  over  the  problem  whether  its  elongated 


GENERAL  STRUCTURE  OF  THE  EARTHWORM    133 

body  and  various  organs  are  the  result  of  its  mode  of  life,  or 
whether  it  adopted  this  mode  of  life  because  of  its  peculiar 
structures.  The  same  problem  recurs  in  connection  with  all 
types  of  living  things  and  may  be  expressed  by  the  question: 
does  the  environment  and  mode  of  life  of  an  animal  type  cause 
the  race  to  become  adapted  to  its  surrounding  conditions,  or 
does  the  animal  type  choose  the  environment  most  suitable  to 
its  peculiar  structures?  We  may  leave  the  discussion  of  this 
question  for  the  present  with  the  non-committal  statement  that 
all  animals  are  more  or  less  perfectly  adapted  to  the  conditions 
of  their  environment,  and  will  take  up  the  problem  of  the 
significance  of  such  adaptations  in  a  later  chapter. 

B.  REGIONAL  DIFFERENTIATION. — A  superficial  examina- 
tion of  the  worm  is  sufficient  to  show  that  it  has  quite  definite 
structures. 

Metamerism. — The  entire  body  is  divided  by  faint  ring-like 
constrictions  or  annuli  into  short  segments  called  somites  or 
metameres,  which  on  first  view  seem  to  be  all  alike.  These 
rings  are  characteristic  of  a  great  group  of  worms  called  An- 
nulata  or  annelids  from  this  peculiarity,  and  all  are  metameric 
animals  in  which  only  slight  modifications  of  the  metameres 
occur.  With  metamerism,  however,  the  possibilities  of  differen- 
tiation are  almost  unlimited,  and  we  find  that  all  of  the  higher 
types  of  animals,  with  the  exception  of  the  phyla  of  the  soft- 
bodied  molluscs  and  spiny-skinned  Echinoderms,  are  built  on 
this  plan  of  structure.  It  is  plainly  evident  in  Crustacea,  in- 
sects, fish,  and  snakes,  but  is  limited  to  the  vertebral  column  in 
the  majority  of  vertebrates. 

Antero-posterior  Differentiation. — Even  in  the  earthworm, 
where  the  metameres  seem  to  be  all  alike,  there  is  some  regional 
differentiation.  If  the  mouth  end  of  the  worm  be  tickled,  it 
will  be  found  to  be  more  sensitive  than  the  middle  region  or 
the  opposite  end.  If  a  bright  light  is  suddenly  thrown  on  the 
mouth  end,  the  worm  will  react  vigorously.  This  end  of  the 
worm,  therefore,  is  more  irritable  than  the  other.  Furthermore, 
at  or  near  this  end  there  are  several  external  openings  of 
internal  organs  not  found  elsewhere;  thus  on  the  eighth,  ninth, 


134 


ORGANS  AND  ORGAN  SYSTEMS 


fourteenth,  and  fifteenth  somites  there 
are  different  openings  of  the  reproductive 
system,  while  several  enlarged  somites 
from  the  twenty-eighth  to  the  thirty- 
seventh,  forming  what  is  called  the  clitel- 
lum,  are  also  associated  with  reproduc- 
tion. The  mouth  end  of  the  earthworm 
is  thus  differentiated  from  the  remainder 
of  the  worm.  We  can  hardly  speak  of  it 
as  the  "head"  end  for  there  is  no  head  nor 
tail,  but  we  speak  of  this  type  of  differ- 
entiation as  antero-posterior  differentia- 
tion, or  anterior  and  posterior  ends.  In 
higher  types  of  animals  this  type  of  dif- 
ferentiation leads  to  very  definite  head 
formation  and  centralization  of  the  nerv- 
ous system,  while  the  posterior  end  al- 
ways bears  the  vent  or  anus  (Fig.  52). 

Dorso-ventral  D ijfe rentiation . — T h e 
worm  always  crawls  on  one  surface.  If 
turned  over  on  its  "back,"  it  objects  vig- 
orously and  quickly  resumes  its  normal 
position.  The  surface  on  which  it  crawls 
also  appears  different  from  the  other;  it  is 
more  flattened;  many  papilla-like  whitish 
glands  are  present,  especially  in  the 
anterior  part,  and  the  various  external 
openings  (mouth,  anus,  reproductive, 
excretory)  are  found  here.  Further- 
more peculiar  bristle-like  setae,  which  can 
be  felt  by  gently  drawing  the  worm 
between  the  fingers,  are  found  on  this 
surface.  There  is,  therefore,  a  fairly 
well-marked  differentiation  between  the 

FIG.  52. — Enlarged  diagram  of  the  anterior  and  posterior  parts  of  the  earth- 
worm as  seen  from  the  ventral  side,  an,  Anus;  c,  clitellum;  g.p..  glandular  swell- 
ings on  the  twenty-sixth  somite;  m,  mouth;  o.d>,  external  openings  of  the  oviducts; 
ps,  prostomium;  s,  setae;1-?.;*.,  openings  of  the  seminal  receptacles;  s.d.,  external 
openings  of  the  sperm  ducts;  1-40,  numbers  of  the  somites  beginning  behind  the 
prostomium.  (From  Sedgwick  and  Wilson.) 


STRUCTURE  OF  THE  EARTHWORM  135 

crawling  surface  or  belly  and  the  opposite  more  rounded  sur- 
face or  back,  and  the  phenomenon  is  called  dorso-ventral  dif- 
ferentiation, which  becomes  more  plainly  marked  in  higher 
types  of  animals. 

Bilateral  Symmetry. — All  of  the  organs  of  the  body  which  do 
not  lie  on  the  median  line  are  found  in  pairs,  one  on  each  side  of 
a  plane  passing  through  the  longitudinal  center  of  the  body. 
The  mouth,  anus,  and  entire  digestive  tract  are  unpaired  and 
lie  in  the  median  plane;  so  do  the  main  blood-vessels,  but  all  of 
the  reproductive  organs,  excretory  organs,  nervous  system,  mus- 
culature, setae,  etc.,  are  paired  structures,  so  that  one  entire 
side  of  the  earthworm  is  an  exact  replica  of  the  other.  This 
phenomenon,  also  characteristic  of  the  higher  animals,  is  called 
bilateral  symmetry. 

External  Apertures. — Some  of  these  are  too  minute  to  be  seen, 
but  others  can  be  easily  made  out.  Two  pairs  of  minute  pores 
(openings  of  the  seminal  receptacles)  are  on  the  ventral  surface 
between  the  gth  and  loth  and  zoth  and  nth  somites;  a  pair  of 
male  genital  openings  are  on  the  i5th  and  a  pair  of  female 
genital  openings  are  on  the  i4th.  On  the  ventral  surface  also 
there  are  two  extremely  minute  openings  of  the  excretory 
organs  (nephridia)  in  each  somite,  except  the  first  three  or 
four  and  the  last.  With  the  exception  of  the  anus,  all  of  the 
openings  posterior  to  the  male  genital  pore  are  too  minute  to 
be  seen. 

While  most  of  the  external  openings  are  on  the  ventral  sur- 
face, some  are  on  the  dorsal  surface.  Here,  for  example,  are  the 
dorsal  excretory  pores,  one  to  each  somite,  after  the  loth,  in 
the  annular  creases,  and  very  difficult  to  see. 

Setae. — There  are  no  true  appendages  on  the  worm's  body, 
but  if  the  animal  is  drawn  gently  through  the  fingers,  fine  bris- 
tle-like structures  may  be  felt.  These  are  setae  or  bristles, 
easily  seen  with  a  hand  lens.  There  are  eight  setae  to  each 
somite, -arranged  in  four  double  rows  on  the  ventral  surface  and 
the  sides.  They  aid  the  worm  in  locomotion  by  catching  into 
the  earth  which  acts  as  a  fulcrum.  The  flattened  tail  of  some 
forms  (Lumbricus  terrestris)  also  serves  a  useful  purpose  in 


136 


ORGANS  AND  ORGAN  SYSTEMS 


anchoring  the  animal  in  its  burrow,  while  the  anterior  end  moves 
freely  about  in  a  small  radius  around  the  hole. 

C.  INTERNAL  STRUCTURE. — A  first  impression  is  that  the 
internal  structures  of  an  earthworm  consist  of  a  tube  within 
a  tube,  the  inner  tube  being  the  alimentary  tract  continuous 
from  the  mouth  at  the  anterior  end  to  the  anus  at  the  posterior 
(Fig.  53).  The  outer  tube,  formed  by  the  body  wall,  is  strictly 
speaking,  not  one  continuous  tube  but  a  multitude  of  minute 


FIG.  53. — General  diagrams  of  the  earthworm  as  seen  in  longitudinal  section. 
A  and  C,  and  transverse  section  B,  showing  the  two  tubes,  the  coelom,  and  the 
dissepiments,  a,  Aortic  loops;  al,  digestive  tract;  an,  anus;  e.g.,  cerebral  ganglia; 
coe,  coelom;  c.v.,  parietal  vessels;  ds,  dissepiments;  d.v.,  dorsal  vessel;  m,  mouth; 
n,  nephridia;  o,  ovary;  o.d.,  oviduct;  s.i.,  ventral  vessel? 
Wilson.) 


(From  Sedgwick  and 


tubes  (150  more  or  less),  formed  by  the  transverse  partitions, 
septa  or  dissepiments,  attached  to  the  body  wall  where  the  annuli 
mark  their  positions.  The  cavities  between  these  dissepiments 
are  termed  coelomic  cavities,  or  simply  the  coelom.  The  body 
wall  is  relatively  thick  and  muscular,  being  made  up  of  epithel- 

FIG.  54. — Anterior  part  of  the  body  of  the  earthworm  as  it  appears  when  the 
dorsal  wall  is  removed,  ao,  Aortic  loops;  ph,  pharynx;  e.g.,  cerebral  ganglia; 
oe,  oesophagus;  s.v.,  seminal  vesicles;  s.r.,  seminal  receptacles;  c.gl.,  calciferous 
glands;  c,  crop;  g,  gizzard;  d,  dissepiment;  s.L,  stomach  intestine;  d.v.,  dorsal 
vessel.  (From  Sedgwick  and  Wilson.) 


STRUCTURE  OF  THE  EARTHWORM  137 


ps 


FIG. 


138  ORGANS  AND  ORGAN  SYSTEMS 

him,  muscles,  nerves,  glands,  connective  tissue,  blood  vessels, 
and  endothelium,  and  the  whole  is  covered  on  the  outside  by  a 
delicate  lifeless  coat  termed  the  cuticle. 

The  Digestive  System. — The  food  of  an  earthworm  consists  of 
leaves,  grass,  animal  tissues  of  any  kind,  and  the  minute  forms 
of  life  found  in  the  ordinary  dirt.  Quantities  of  this  dirt  are 
continually  taken  in  by  the  animal  and  are  passed,  unaltered, 
through  the  alimentary  tract  to  be  defecated  through  the  anus. 

The  apparatus  for  food  digestion  and  absorption  is  much  more 
complicated  than  in  Hydra,  where  digestion  is  largely  intracellu- 
lar.  In  the  worm,  it  is  inter-cellular  and  occurs  in  cavities,  into 
which  the  lining  cells  secrete  digestive  ferments.  The  anterior 
end  of  the  digestive  tract  is  covered  by  a  thick  muscular  wall. 
This  part,  the  pharynx  (Fig.  54,  ph),  is  used  as  a  sucking  mech- 
anism for  drawing  in  food  matters.  Posterior  to  the  pharynx  is 
the  oesophagus,  a  thin-walled  tube  extending  from  about  the 
sixth  or  seventh  somite  to  the  fourteenth  or  fifteenth,  covered 
over  from  the  seventh  or  eighth  to  the  fifteenth  by  the  large 
yellowish  vesicles  of  the  reproductive  system,  and  encircled 
by  the  fine  " aortic  arches"  of  the  blood  vascular  system.  Pos- 
terior to  the  reproductive  organs  and  on  the  ventral  side  of  the 
oesophagus  are  three  pairs  of  bright  yellow  organs  called  the 
calciferous  glands,  the  secretions  of  which  serve  to  neutralize  the 
acids  taken  in  with  the  food.  Immediately  behind  the  calci- 
ferous glands  the  alimentary  tract  expands  in  to.  a  larger  thin- 
walled  pouch,  termed  the  crop,  which  serves  as  a  food  reservoir. 
'The  crop  opens  into  a  thick-walled  reservoir  or  muscular  pouch, 
called  the  gizzard,  where  the  food  materials  are  ground  up  into 
fine  particles,  the  dirt,  sand  grains,  etc.,  serving  a  useful  pur- 
pose in  the  process.  Posterior  to  the  gizzard,  from  about  the 
twenty-sixth  somite  to  the  posterior  end  of  the  worm,  the  diges- 
tive tract  consists  of  a  uniform  tube  lined  by  secreting  cells. 
This  tube  is  called  the  stomach-intestine  from  its  combined  func- 
tions, and  it  is  covered  with  a  thick  layer  of  brownish-yellow 
glandular  cells  termed  the  chlorogogue  cells.  These  are  richly 
supplied  with  blood  vessels,  and  are  supposed  to  have  some  func- 
tion connected  with  excretion.  Finally,  at  the  posterior  end, 


DIGESTIVE  SYSTEM  OF  THE  EARTHWORM       139 

the  stomach-intestine  opens  to  the  outside  through  the  anus  by 
a  short  section  of  non-functional  tissue,  called  the  procto daeum. 

In  the  cavities  between  the  digestive  tract  and  the  body  wall 
lie  all  of  the  other  important  organs  of  the  worm.  They  are, 
therefore,  morphologically  speaking,  inside  the  worm  while 
undigested  food,  dirt,  etc.,  although  inside  the  digestive  tract, 
are,  morphologically,  outside  of  the  animal.  Some  of  these 
internal  organs,  like  those  of  the  excretory  system,  are  repeated 
in  each  somite;  others,  like  the  blood,  vascular  and  nervous 
systems,  are  continuous  from  one  end  of  the  body  to  the  other, 
while  still  others,  like  the  reproductive  system,  are  concentrated 
in  one  part,  occupying  only  a  few  somites. 

D.  PHYSIOLOGY  OF  THE  DIGESTIVE  SYSTEM.  Buccal  Cavity 
and  Pharynx. — The  mouth  is  covered  by  a  dorsal  prolongation 
of  the  first  somite,  functioning  as  an  upper  lip  or  prostomium. 
A  much  smaller  under  lip  completes  the  border.  The  buccal 
cavity  is  a  relatively  spacious  hollow  reaching  as  far  as  the 
third  somite,  and  its  wails  are  provided  with  special  muscles 
reaching  to  the  body  wall.  This  cavity  opens  into  a  larger  and 
more  muscular  pharynx.  The  lips  do  not  act  as  grasping  organs 
for  ingestion  of  leaves,  but  the  prostomium  is  rather  an  organ  of 
smell,  while  the  pharynx  with  its  heavy  muscles  acts  as  a  suc- 
tion pump  for  drawing  leaves  into  the  burrows  and  for  ingesting 
them  afterward.  The  leaves  line  the  walls  of  the  burrows  or 
partly  extend  out  of  them,  masking  the  openings.  If  such  a 
partially  visible  leaf  is  pulled  out,  the  part  inside  the  burrow  will 
be  found  to  be  a  mere  skeleton,  the  mesophyll  structures  having 
been  sucked  into  the  pharynx  of  the  worm  by  the  action  of  the 
muscular  pump.  This  action  is  facilitated  by  the  secretion 
from  the  mouth  and  pharynx  of  a  digestive  fluid  of  alkaline 
reaction  which,  however,  only  softens  and  does  not  digest, 
although  Darwin  suspected  the  presence  of  tryptic  protease  and 
amylolytic  ferments. 

The  Oesophagus. — The  oesophagus  stretches  from  about  the 
sixth  to  the  fourteenth  somites;  it  is  somewhat  laterally  com- 
pressed with  longitudinal  and  annular  muscles.  In  the  posterior 

part  of  the  oesophagus  the  walls  are  considerably  thickened  to 
10 


140  ORGANS  AND  ORGAN  SYSTEMS 

form  three  pairs  of  peculiar  calciferous  glands.  These  glands 
consist  of  numerous  flat  and  broad  pockets  of  tissue  radially 
arranged  on  the  oesophagus  as  axis.  The  flattened  pockets  are 
enclosed  in  a  muscular  sheath  and  lie  in  a  blood-filled  sinus, 
while  between  the  pockets  are  collections  of  lime.  These 
" glands"  are- not  true  glandular  diverticula  of  the  oesophagus, 
but  are  mesodermal  in  origin  and  are  merely  the  walls  of  the 
blood  vessels.  The  cells  of  the  pockets  take  crystals  of  calcium 
carbonate  from  the  blood  and  secrete  them  in  a  milky  fluid 
into  the  oesophagus.  (See  Combault,  Harrington,  etc.) 

The  function  of  the  glands  is  not  entirely  clear,  although  several 
assumptions  have  been  made  which  are  more  or  less  well  grounded. 
Claparede  and  Darwin  believed  that  the  milky  fluid  may  be  an  excretion 
of  the  great  quantity  of  lime  which  is  contained  in  fallen  leaves  and 
accumulates  in  the  blood  after  digestion  of  the  leaves.  Harrington,  on 
the  other  hand,  found  that  secretion  of  lime  from  the  glands  diminishes 
if  the  worms  are  fed  with  calcium  carbonate,  but  increases  if  fed  with 
acidified  food,  and  he  accepts  a  second  hypothesis  of  Darwin's,  viz.,  that 
the  lime  plays  a  role  in  digestion.  This  role  is  to  neutralize  the  humus 
acids  contained  in  decomposing  vegetation,  and  to  prepare  a  suitable 
alkaline  medium  for  the  action  of  tryptic  ferments. 

A  third  view  advanced  by  Combault  is  that  the  calciferous  glands 
form  a  sort  of  internal  breathing  organ  for  removing  CO2  from  the  blood, 
combining  it  with  calcium  and  excreting  it  as  lime  into  the  oesophagus. 
This  view,  which  is  also  supported  by  experimental  evidence,  does  not 
exclude  the  possibility  of  a  digestive  function,  but  if  true,  it  indicates  the 
further  function  of  preventing  a  surplus  of  CO 2  in  the  blood. 

The  Crop  and  Gizzard. — In  the  i4th  segment  the  digestive 
tract  enlarges  to  form  a  thin-walled  expansion  called  the  crop, 
extending  from  the  i4th  to  the  i6th  somites.  No  special  func- 
tion, apart  from  storage,  is  attributed  to  this  organ,  but  it  opens 
directly  into  a  thick-walled  gizzard  provided  with  powerful 
circular  muscles.  The  contraction  of  these  muscles,  acting  on 
the  contained  food  material  mixed  with  gravel,  results  in  the 
trituration  of  the  solid  food  materials  and  prepares  them  for 
digestion  in  the  stomach  intestine. 

The  Stomach  Intestine. — This,  the  most  important  organ  of 
the  alimentary  system,  begins  at  about  the  i8th  somite  and  runs 


DIGESTIVE  SYSTEM  OF  THE  EARTHWORM       141 

in  a  straight  course  to  the  posterior  end  of  the  worm.  It  consists 
of  epithelial,  vascular,  circular  and  longitudinal  muscular  tissues 
and  is  covered  on  the  outside  by  peculiar  yellowish-brown 
chlorogogue  cells,  derived  from  the  coelomic  endothelium.  Along 


FIG.  55. — Stereogram  showing  the  relations  of  organs  in  the  posterior  part  of 
the  earthworm.  (Worked  out  by  Professors  McGregor  and  Calkins,  and  drawn 
by  Miss  Hedge.) 

the  dorsal  median  line  a  longitudinal  fold  of  tissue  coming  from 
the  dorsal  wall  of  the  intestine  runs  the  entire  length  of  this 
organ  as  far  as  the  rectum.  This  fold,  called  the  typhlosole 
(Fig.  55),  has  a  different  form  in  different  regions  of  the  body 
and  contains  additional  blood  vessels  and  chlorogogue  cells, 
thus  increasing  the  area  of  the  digestive  surface.  The  intestine, 


142  ORGANS  AND  ORGAN  SYSTEMS 

finally,  is  constricted  at  each  dissepiment  so  that  its  structure 
follows  the  general  metamerism  of  the  body. 

The  digestive  fluid  secreted  by  the  wall  cells  of  the  intestine 
corresponds  in  its  essential  features  with  the  pancreatic  juice 
of  mammals.  Free  acids  cannot  be  detected  in  the  gut, 
where  the  fluids  in  general  show -a  slightly  alkaline  reaction. 
According  to  Lesser  and  Taschenberg,  albumin  is  broken  down 
under  action  of  this  digestive  fluid  in  3  1/2  hours  at  37°  C. 
if  the  medium  is  slightly  alkaline,  and  in  28  1/2  hours  if  it  is 
slightly  acid.  According  to  Abderhalden  and  Heise,  a  pep- 
togenic  ferment  is  also  present,  which  accounts  for  the  slow 
digestion  in  an  acid  medium.  The  same  observers  also  ex- 
tracted an  amylolytic  ferment  which 
changes  starch  into  sugar  (maltose), 
and  found  traces  of  a  fat  emulsify- 
ing ferment. 

The  digestive  ferments  are  se- 
creted by  gland  cells  distributed 
among  absorbing  cells  of  the  gut 
epithelium  (Fig.  56).  In  prepared 

seetions'  they  ma7 be  distinctly made 

enlarged   gland   cells   with   se-     out,     if    filled    With    granules    which 
cretions.     and   between  them,     ,  ,,  ,      ,     ... 

the    absorptive  cells.    (From    have  an  albuminous  nature,  but  if 
K.  C.  Schneider.)  emptied  of 'granules,  they  become  so 

small  and  compressed  that  they  are  difficult  to  find. 

The  absorption  cells  are  columnar,  ciliated,  epithelial  cells, 
somewhat  broader  at  the  ciliated  end.  In  each  there  is  a 
typical  and  characteristic  closing  apparatus.  The  free  sur- 
face possesses  a  cuticle-like  covering  which  bears  a  hedge  of 
fine,  stiff  rods,  through  which  the  cilia  pass  from  their  basal 
bodies  in  the  cell  to  the  lumen  of  the  gut.  These  cilia  are 
absent  on  the  cells  of  the  typhlosole,  where  fat  absorption  is  the 
chief  role  (Greenwood).  The  function  of  the  minute  rods'is  un- 
known but  they  occur  very  generally  on  absorption  cells. 
Granules  of  absorbed  food-stuffs  are  often  visible  in  these  cells, 
some  of  which  may  be  recognized.  Thus,  if  powdered  carmine 
or  indigo  is  mixed  with  the  worm's  food,  the  colored  granules 


VASCULAR  SYSTEM  OF  THE  EARTHWORM        143 


\ 


V 


144  ORGANS  AND  ORGAN  SYSTEMS 

reappear  in  the  absorptive  cells;  chlorophyll  and  fat  have  also 
been  detected. 

The  Rectum. — The  rectum  of  the  earthworm  is  the  posterior 
part  of  the  intestine,  which  bears  no  typhlosole.  It  opens  to  the 
outside  through  the  anus  which  is  provided  with  a  sphincter 
muscle.  According  to  Hensen,  each  worm  deposits  1/2  gram 
of  faeces  in  24  hours,  and  these  faeces  make  up  the  castings 
shown  by  Darwin  to  have  great  economic  importance. 

Further  Fate  of  the  Absorbed  Nutriment. — The  gut  walls  of  the 
earthworm  are  richly  supplied  with  blood  vessels  and  finer 
capillaries  which  form  a  vascular  network  throughout  the  entire 
intestine.  It  is  highly  probable,  although  never  demonstrated, 
that  the  absorbed  food  material  is  passed  directly  into  the  blood 
through  the  walls  of  these  vessels. 

E.  BLOOD  VASCULAR  SYSTEM. — In  the  earthworm  it  is  the 
plasm  or  fluid  that  is  colored  red  by  haemoglobin,  while  the 
living  cells  of  the  circulation  are  colorless.  As  in  vertebrates, 
the  blood  flow  is  continuous,  the  circulation  being  closed 
throughout,  i.e.,  the  blood  is  always  in  tubular  vessels.  The 
main  trunks  are  (i)  a  dorsal  vessel  lying  on  the  digestive  tract 
and  giving  off  in  the  anterior  segments  five  pa,irs  of  enlarged 
vessels,  the  so-caHed  "aortic  arches"  or  loops.  These  loops 
are  connected  below  the  oesophagus  with  (2)  a  ventral  vessel 
also  running  the  entire  length  of  the  animal  below  the  digestive 
tract.  Both  dorsal  and  ventral  vessels  give  off  branches  to  the 
adjacent  tissues.  One  pair  (parietals)  run  from  the  dorsal 
vessel  in  each  somite  and  along  the  dissepiment  to  the  body  wall, 
where  they  split  into  numerous  fine  brandies  (capillaries),  pene- 
trating the  dermal  musculature  and  the  epithelium.  Other 
branches  of  the  dorsal  vessel  are  given  off  to  the  digestive  tract, 
ending  in  capillaries  in  the  walls  of  the  stomach-intestine  and 
other  organs.  In  the  anterior  region  two  lateral  vessels  are 
given  off,  which  supply  the  reproductive  organs.  Three  other 
longitudinal  ventral  vessels  run  the  length  of  the  worm;  one, 
the  sub-neural  vessel,  lies  below  the  nerve  cord,  while  two 
others,  lateral-neurals,  are  embedded  in  the  connective  tissue 
about  the  nerve  cord,  one  on  each  side  (Figs.  55  and  57). 


VASCULAR  SYSTEM  OF  THE  EARTHWORM       145 

Vascular  Circulation. — The  circulation  of  the  blood  is  brought 
about  by  peristalsis  or  the  consecutive  contraction  of  the  circular 

•^^^-x^-*^*-*'***'*^- 

muscles  in  the  walls  of  the  blood  vessels.  This  wave  of  con- 
traction proceeds  in  the  dorsal  vessel  from  the  posterior  end 
toward  the  anterior,  the  blood  being  forced  ahead  of  the  wave 
of  contraction,  as  one  might  force  water  from  a  rubber  tube. 
The  details  of  the  path  followed  in  the  circulation  of  the  blood 
are  not  fully  known,  but  there  is  abundant  evidence  that  the 
essential  features  in  the  following  account  are  correct  (cf. 
Figs.  55  and  57). 

The  blood  passes  into  the  pharyngeal  vessels  anterior  to  the 
aortic  loops;  also  through  the  aortic  loops  into  the  ventral 
vessel,  where  the  flow  is  from  the  anterior  toward  the  posterior 
end.  From  the  ventral  vessel  branches  are  given  off  to  the 
digestive  tract,  the  nephridia,  and  the  body  wall.  In  the  di- 
gestive tract  these  vessels  branch  repeatedly  until  they  result  in 
fine  capillaries  running  throughout  the  vascular  area  of  the  di- 
gestive system.  Similar  capillaries  connect  with  these  and 
conduct  the  blood,  now  loaded  with  products  of  digestion,  into 
larger  vessels -which,  in  turn,  open  into  the  dorsal  vessel.  It  is 
possible  that  the  waste  matters  (urea),  contained  in  the  blood 
thus  directed  into  the  digestive  system,  are  disposed  of  through 
the  agency  of  the  chlorogogue  cells.  The  blood  vessels  which 
enter  the  nephridia  likewise  break  up  into  capillaries  where  the 
blood  probably  gives  up  its  urea.  The  purified  blood  then  passes 
into  the  parietal  vessels,  or  into  the  body  wall,  and  is  ultimately 
conducted  back  to  the  dorsal  vessel.  In  the  same  way,  the 
vessels  which  enter  the  body  wall  ultimately  end  in  capillaries 
distributed  throughout  the  general  surface  of  the  body.  Here 
the  blood  loses  its  €62,  and  takes  in  oxygen,  and  is  then  carried 
through  progressively  larger  vessels  back  to  the  lateral-neural 
vessels,  from  which  it  passes  directly  to  the  sub-neural  vessel, 
and  then,  by  way  of  the  parietals,  back  to  the  dorsal  vessel. 
Within  each  of  the  parietal  vessels,  near  its  point  of  union  with 
•  the  dorsal  vessel,  is  a  conspicuous  valve  which  \rnay  be  seen  in 
the  Jiving  worm.  This  valve  is  so  placed  that  blood  can  flow 
from  the  parietal  into  the  dorsal  vessel  after  a  peristaltic  wave 


146  ORGANS  AND  ORGAN  SYSTEMS 

.  has  passed,  while  blood  is  prevented  from  flowing  back  into  the 
parietal  vessel  by  closure  of  the  valve,  due  to  pressure  of  the 
oncoming  peristaltic  wave. 

The  dorsal  vessel  thus  functions  like  a  heart,  the  force  neces- 
sary to  propel  the  blood  through  the  large  vessels  and  capillaries 
coming  from  the  consecutive  contractions,  or  peristalsis,  of  its 
circular  muscles,  and  the  force,  in  turn,  comes  from  the  trans- 
formed energy,  due  to  oxidation,  in  the  muscle  cells. 

Coelomic  Circulation. — Another  circulating  fluid  is  contained 
in  the  body  cavity,  or  coelom,  which  is  continuous  throughout 
all  of  the  somites  of  the  worm  through  dorsal  apertures,  in  the 
form  of  slits,  between  the  dissepiments  and  the  digestive  tract. 
This  fluid  is  made  up  of  a  colorless  plasm  with  white  blood  cells 
or  leucocytes.  It  is  washed  back  and  forth  by  movements  of 
the  worm,  thus  bathing  the  endothelium  lining  the  coelom,  but 
there  is  no  definite  circulation. 

F.  THE  EXCRETORY  SYSTEM. — Nephridia. — The  waste  mat- 
ters of  metabolism  are  disposed  of  through  the  action  of  small 
but  complicated  organs,  called  nephridia,  a  pair  of  which  may  be 
found  in  all  of  the  somites  after  the  first  four.  Each  nephridium 
consists  of  similar  parts,  the  most  important  of  which  are:  (i) 
the  funnel  or  nephrostome,  (2)  the  ciliated  neck,  (3)  the  coiled 
narrow  tube,  (4)  the  wide  glandular  tube,  and  (5)  the  ejacula- 
tory  duct  opening  to  the  outside  (Fig.  58). 

The  ciliated  neck  of  the  nephrostome  passes  through  the 
anterior  wall  of  the  somite,  close  to  the  mid-ventral  line.  The 
nephrostome,  therefore,  lies  in  the  somite  anterior  to  the  one 
containing  its  own  nephridium,  so  that  waste  matters  of  anyone 
somite  are  expelled  to  the  outside  by  the  nephridium  of  the  next 
posterior  somite.  The  nephrostomes  or  mouths  of  the  nephridia 
"are  flattened  fan-like  structures,  consisting  of  two  flattened 
lamellae  or  plates  with  a  narrow  slit-like  opening  between  them ; 
the  great  cells  lining  the  opening  are  covered  with  powerful  cilia 
which  maintain  a  constant  current  toward  the  tubular  part  of 
the  nephridium.  These  tubes  are  developed  in  coils  which  lie 
in  the  posterior  parts  of  the  somites,  three  coils  or  turns  in  each, 
the  third  ending  in  an  enlarged  portion  opening  to  the  outside 


MUSCLES  OF  THE  EARTHWORM 


147 


on  the  ventral  wall  of  the  somite  (Figs.  55  and  58).     All  of  the 
turns  are  richly  supplied  with  blood  vessels. 

If  carmine  powder  is  injected  into  the  coelom,  it  is  taken  up 
by  the  chlorogogue  cells,  which  then  break  down,  freeing  the 
carmine  together  with  fragments  of  the  chlorogogue  cells,  and 
all  are  caught  up  by  the  current  made  by  the  nephrostome,  and 
carried  through  the  nephridium  to  the  outside.  From  this 
experiment  the  conclusion  has  been  drawn  that  some,  at  least,  of 


JT 


FIG.  58. — Nephridium  of  Lumbricus.  /,  Funnel  or  nephrostome;  ds,  dissepi- 
ment; n.t.,  narrow  tube,  ciliated  between  a  and  b,  d  and  e  and  at  c\  m.t.,  middle 
tube  ciliated  between  h  and  z;  w.t.,  wide  tube;,w./>.,  muscular  part;  ex,  external 
opening.  (From  Sedgwick  and  Wilson,  after  Benham.) 

the  waste  matters  of  the  tissues  are  brought  in  the  circulation 
to  the  chlorogogue  cells,  and  are  acted  upon  by  the  fluids  of  these 
cells.  The  products  of  this  activity  are  liberated  into  the  coe- 
lom  by  the  fragmentation  of  the  cells,  and  then  are  excreted  from 
the  worm  by  the'nephridia. 

Dorsal  Pores. — Excretion  is  also  carried  on  to  a  limited  extent 
through  dorsal  pores  situated  in  the  annuli  in  the  mid-dorsal 
line. 

G.  THE  MUSCULAR  SYSTEM. — The  main  muscular  and  sup- 
porting system  of  the  earthworm  is  relatively  simple,  consisting 
of  two  walls  of  muscle  fibers  which  form  a  continuous  sheath  from 
anterior  to  posterior  ends  of  the  worm.  Being  united  with  the  skin 
to  form  the  body  wall,  it  is  known  as  dermal  musculature.  The 


148 


ORGANS  AND  ORGAN  SYSTEMS 


inner  wall  consists  of  longitudinal  fibers  running  from  somite  to 
somite,  and  their  contraction  results  in  drawing  head  and 
tail  end  together,  or  in  shortening  the  worm  (Fig.  59).  .The 
muscle  fibers  are  closely  packed  together,  giving  the  appear- 
ance of  many  muscles.  The  outer  wall  consists  of  fibers 
running  around  the  somite  at  right  angles  to  the  longitudinal 


m 


.m 


FIG.  59. — Transverse  section  of  the  earthworm  behind  the  clitellum.  a.c., 
Cavity  of  the  digestive  tract;  c,  cuticle;  coe,  coelom;  c.m.t  circular  muscles;  c.vty 
parietal  vessel;  d.v.,  dorsal  vessel;  hy,  hypodermis;  l.m.,  longitudinal  muscles; 
n.c.,  ventral  nerve  chain;  p.e.,  peritoneal  endothelium;  s,  seta;  s.g.,  setigerous 
gland;  s.i.v.,  ventral  vessel;  s.m.,  muscle  connecting  two  groups  of  setae  on  the 
same  side;  ty,  typhlosole.  (From  Sedgwick  and  Wilson.) 

fibers,  and  their  contraction  results  in  shortening  the  diameter 
of  the  somite  and  thus  in  elongating  the  worm.  Both  sheaths 
of  muscle  are  broken  in  four  places  at  points  where  setae  are 
formed,  and  here  special  muscles  for  moving  the  inner  ends 
of  the  setae  are  developed;  by  their  contraction  the  setae  are 
moved  one  way  or  another.  These  setae  are  lifeless  rods  of 
chitin,  somewhat  sharpened  at  the  outer  ends  and  formed  from 


NERVOUS  SYSTEM  OF  THE  EARTHWORM        149 

special  glands  termed  the  seta-sacs.  These  are  often  of  large 
size,  and  are  conspicuous  when  the  worm  is  opened.  Between 
the  two  setae  of  each  pair,  a  few  longitudinal  bundles  of  muscle 
fibers  help  to  strengthen  the  body  wall  and  to  complete  the  mus- 
cular sheath,  while  smaller  muscles  connect  the  adjacent  setae 
on  each  side  (Fig.  59  s.m.). 

Other  special  muscles  form  the  walls  of  the  pharynx,  and  by 
their  contraction  and  relaxation  they  shut  and  open  this  organ, 
thus  making  it  a  sucking  pouch  which  draws  in  dirt,  leaves,  and 
other  extraneous  matters. 

Still  other  special  muscles  form  the  walls  of  the  gizzard,  mak- 
ing it  a  grinding  organ  for  cutting  up  food  received  from  the 
crop.  Circular  and  longitudinal  muscles  also  form  part  of  the 
wall  of  the  stomach  intestine,  and  by  their  successive  contrac- 
tion force  the  enclosed  undigested  food  materials  toward  the 
anus,  thus  acting  by  peristalsis  as  do  the  blood-vessel  muscles 
which  form  a  part  of  the  walls. 

H.  THE  NERVOUS  SYSTEM. — The  nervous  system  is  closely 
connected  with  the  muscular  system,  and  it  is  well  to  get  in 
mind  the  muscle-nerve  combination,  for  one  always  involves  the 
other,  sensory  cell,  central  nervous  system,  and  muscle  all 
working  together  in  what  is  termed  a  "reflex  action." 

The  Sensory  System. — The  cells  of  the  skin  are  of  different 
kinds,  the  majority  being  epithelial  or  columnar  cells,  with 
secreting  or  goblet  cells  interspersed  here  and  there,  which  form 
the  slimy  secretion  poured  out  when  the  worm  is  irritated.  In 
addition  to  these,  there  are  many  small  sensory  cells  which 
receive  stimuli  when  the  worm  is  irritated.  These  are  much 
more  numerous  in  the  anterior  part  of  the  worm  than  elsewhere, 
making  this  the  most  sensitive  or  irritable  portion  of  the  whole 
body.  The  irritation  received  by  these  sensory  cells  is  passed 
as  a  nervous  impulse  through  prolongations  of  the  sensory  cells 
in  the  form  of  nerve  fibers  to  the  central  nervous  system  which 
runs  from  end  to  end  of  the  worm.  There  are  no  aggregates  of 
sensory  cells  to  form  sense  organs,  but  the  entire  skin  is  sensitive. 

The  earthworm  marks  a  great  advance  in  the  organization  of 
the  nervous  system.  In  Hydra  and  the  coelenterates  there  is 


150  ORGANS  AND  ORGAN  SYSTEMS 

nothing  like  the  highly  co-ordinated  nervous  system  of  the 
annelids  and  higher  types  generally.  Nerve  cells  are  present,  it 
is  true,  in  Hydra,  and,  as  we  have  seen,  they  form  a  more  or  less 
complete  nervous  network  throughout  the  organism,  but  they 
consist  of  sensory  cells  and  isolated  nerve  cells  whose  processes 
connect  with  the  equally  isolated  epithelio-muscle  cells. 

In  the  earthworm,  in  common  with  all  higher  animals,  the 
sensory  cells  do  not  connect  in  this  direct  manner  with  the  mus- 
cles, but  act  through  a  central  nervous  system.  The  condition 
in  Hydra  may  be  compared  with  a  primitive  telephone  system, 
where  everyone  rushes  to  the  phone  whenever  a  bell  rings  in  the 
system,  while  the  earthworm  condition  may  be  compared  with  a 
well-equipped  and  efficient  modern  telephone  plant,  where  all 
peripheral  calls  are  sent  directly  to  the  central  exchange  and 
there  properly  classified  and  transmitted.  We  distinguish, 
therefore,  two  distinct  parts  of  the  nervous  system  of  the  earth- 
worm (i)  the  sensory  or  peripheral  system,  described  above, 
and  (2)  the  central  nervous  system. 

The  Central  Nervous  System. — The  central  nervous  system 
consists  of  a  double  nerve  cord  lying  below  the  digestive 
tract,  with  a  large  double  swelling  in  each  somite.  These 
swellings  are  termed  ganglia,  and  they  are  made  up  of  masses 
of  nerve  cells.  The  ganglia  are  connected  from  somite  to  somite 
by  the  heavy  double  nerves  termed  commissures,  so  that  the 
entire  worm  is  bound  together  by  a  continuous  system  of  com- 
missures and  ganglia  which  form  a  fairly  homogeneous  central 
nervous  system  (Fig.  60). 

The  central  nervous  system  is  connected  with  the  sensory 
cells  of  the  body  wall,  and  with  all  of  the  organs  of  the  somites, 
by  three  pairs  of  nerves  which  leave  the  ganglia  as  shown  in 
Fig.  60.  These  nerves  are  supported  by  the  dissepiments  and 
body  wall  and  branch  and  sub-branch,  until  they  are  lost  in  a 
network  of  fine  fibers  penetrating  muscles  and  epithelium. 
Each  of  the  main  nerves  is  a  bundle  of  fine  fibers  which  transmit 
sensory  impulses  to  the  ganglia,  and  motor  impulses  from  the 
ganglia. 

At  the  anterior  end  of  the  ventral  nerve  chain  there  is  a  pair 


NERVOUS  SYSTEM  OF  THE  EARTHWORM        151 


jut 


FIG.  60. — Anterior  part  of  the  worm  laid  open  to  show  the  ventral  organs, 
c.c.,  Circum-oesophageal  commissures;  e.g.,  cerebral  ganglia;  <fo.,  dissepiment; 
/,  funnel  of  nephridium;  np,  nephridium;  o,  ovary;  od,  oviduct;  ph,  pharynx; 
/>5,  prostomium;  f.5.,  seminal  receptacle;  s.d.,  sperm  duct;  s.f.,  sperm  funnel; 
s.v.L,  lateral  seminal  vesicle;  /,  testis;  v.g.  and  v.n.c.,  ventral  nerve  cord.  (From 
Sedgwick  and  Wilson.) 


152  ORGANS  AND  ORGAN  SYSTEMS 

of  larger  ganglia,  known  as  the  sub-oesophageal  ganglia.  The 
commissures  from  them  pass  around  the  oesophagus,  one  on 
each  side,  and  connect  with  a  pair  of  ganglia  in  the  peris- 
tomium,  dorsal  to  the  mouth.  Because  of  their  course  around 
the  oesophagus,  these  commissures  are  called  the  circum- 
oesophageal  commissures,  and  the  two  dorsal  ganglia  are  called 
the  cerebral  ganglia  (c.c.  and  e.g.).  This  pair  represents  the  only 
morphological  element  comparable  with  a  brain  of  higher  ani- 
mals, but  it  is  probable  that  they  have  no  functions  different 
from  those  of  the  ordinary  ganglia  of  the  ventral  chain.  As 
there  are  more  sensory  cells  in  the  anterior  region  of  the  worm, 
it  is  probable,  however,  that  their  functions  are  more  fre- 
quently called  into  play,  so  that  it  is  a  more  active  organ  than 
any  of  the  other  ganglia. 

A  Reflex  Action. — Consciousness,  as  we  understand  it  in 
human  beings,  probably  does  not  exist  in  the  earthworm,  but 
the  relation  between  nervous  impulse  and  muscular  response 
is  so  delicately  adjusted  that  movements  are  produced,  which 
in  human  beings  we  would  interpret  as  conscious  acts.  The 
complicated  movements  of  a  worm,  in  its  efforts  to  free  itself 
from  some  irritating  environment,  may  all  be  traced  back  to  a 
relatively  simple  series  of  processes  termed  a  reflex  action  (Fig. 
61).  Each  reflex  action  involves  five  distinct  elements  of  the 
nervous  system:  (i)  A  sensory  cell  which  receives  the  stimulus 
from  the  outside;  (2)  a  nerve  fiber  bearing  the  sensory  or  affer- 
ent impulse  transmitted  from  the  sensory  cell;  (3)  a  central 
nerve  cell  in  the  ganglion  which  receives  the  sensory  impulse, 
and  transforms  it  into  a  motor  or  efferent  impulse,  which  now 
travels  over  (4)  an  efferent  or  motor  nerve  to  (5)  a  muscle  cell. 
The  stimulus,  thus  conveyed,  starts  contraction  in  the  muscle. 
It  is  probable  that  other  centers  of  activity  are  stimulated,  and 
that,  by  nerve  cells  and  their  processes,  the  nerve  fibers  trans- 
mit impulses  from  one  ganglion  to  another  along  the  entire 
course  of  the  central  nervous  system.  This  is  probable  be- 
cause of  the  presence  in  the  ventral  nerve  chain  of  different 
kinds  of  nerve  fibers,  some  of  -which  originate  in  the  ventral 
ganglia  and  send  processes  in  both  directions  along  the  ventral 


A  REFLEX  ACTION 


153 


cord,  without  leaving  the  cord  at  any  point.  Such  cells  and 
fibers,  known  as  co-ordinating  cells,  extend  from  somite  to 
somite,  and  give  rise  to  co-ordinating  impulses  which  cause  the 
entire  worm  to  act  as  a  single  unit. 

The  ventral  cord  and  ganglia  have  other  cells  than  those 
mentioned,  which  may  be  grouped  together  as  (i)  afferent  nerve 
cells  which  receive  impulses,  (2)  efferent  nerve  cells  which  trans- 


FIG.  61. — Portion  of  a  transverse  section  of  the  ventral  part  of  the  body  of 
Lumbricus,  to  show  the  nerve  connections,  n.c.,  Ventral  ganglion  giving  off  a 
lateral  nerve  l.n.\  p.e.,  peritoneal  endothelium;  l.m.,  longitudinal  muscles;  hy, 
hypodermis;  in  the  nerve  l.n.  are  sensory  fibers  proceeding  inward  from  the 
sensory  cells  (in  black)  of  the  hypodermis,  and  terminating  in  branching  extremi- 
ties; s,  seta.  (From  Sedgwick  and  Wilson,  after  Lenhossek.) 


mit  impulses  to  motor  fibers,  (3)  co-ordinating  cells  which  bring 
about  concerted  action  of  the  entire  chain  of  somites,  (4)  giant 
fibers,  the  functions  of  which  are  somewhat  problematical, 
but  which  may  have  both  supporting  and  co-ordinating  func- 
tions, and  (5)  glia  cells  which  form  the  matrix  or  main  body  of 
the  chain  (Fig.  62). 

Each  young  nerve  cell  in  development  first  forms  an  axial 
process  called  the  axon,  which  carries  impulses  away  from  the 
cell.  Other  processes  of  the  cell  are  termed  dendrites,  which 
are  shorter  and  more  branched  than  the  axon,  and  they  receive 


154 


ORGANS  AND  ORGAN  SYSTEMS 


FIG.  62. — Portion  of  the  ventral  nerve  cord  with  two  ganglia  and  six  pairs  of 
lateral  nerves.  The  sensory  nerves  end  in  terminal  branches  which  connect  with 
the  dendrites  of  the  motor  and  co-ordinating  nerve  cells  of  the  cord.  (From 
Retzius.) 


REPRODUCTIVE  SYSTEM  OF  THE  EARTHWORM     155 

and  carry  impulses  to  the  cell.  The  entire  system  of  cell  body, 
axon,  and  dendrites  constitutes  a  nerve  unit  called  the  neuron. 

Every  neuron  is  separate  and  distinct,  and  adjacent  neu- 
rons, while  in  contact,  do  not  fuse;  impulses,  however,  are 
transmitted  from  one  to  another  by  contact. 

The  muscular  and  nervous  systems  are  sometimes  grouped  to- 
gether as  organs  of  relation,  since  it  is  through  them  that  the 
organism  is  in  touch  with  its  environment.  These  systems, 
together  with  those  of  nutrition,  circulation  and  excretion  are 
organs  of  the  individual,  and  have  to  do  only  with  one  animal. 
One  other  system  of  organs — the  reproductive  organs — has 
little  to  do  with  the  individual,  since  it  consists  of  organs  having 
nothing  to  do  with  metabolism,  secretion,  excretion  or  nervous 
response,  but  is,  primarily,  a  system  of  organs  of  the  race,  with 
the  one  common  function  of  perpetuating  the  species  by  re- 
production of  the  same  kind  of  worm.  Hence  we  consider  it 
separately. 

/.  THE  REPRODUCTIVE  SYSTEM. — Like  Hydra  and  the  fern, 
the  earthworm  is  hermaphrodite,  having  both  male  and  female 
organs  of  reproduction.  These  are  somewhat  complicated,  and 
involve  two  sets  of  structures,  one  set  for  the  receiving  and 
storing  of  spermatozoa  received  from  another  worm  during 
copulation,  the  other  set  for  the  manufacture,  development  and 
emission  of  the  mature  spermatozoa  and  eggs. 

The  receptive  organs,  termed  seminal  receptacles  (Fig.  63), 
consist  of  two  pairs  of  globular  sacs  in  the  Qth  and  loth  somites, 
with  openings  to  the  outside  on  the  ventral  surface.  They 
are  small  spherical  sacs  close  to  the  dissepiments,  and  one 
pair,  at  least,  may  be  hidden  by  the  overlying  seminal  vesicles 
or  organs  for  the  manufacture  of  spermatozoa.  At  the  period 
of  maturity,  these  receptacles  are  usually  filled  with  mature 
spermatozoa  which  have  been  formed  in  another  worm.  When 
the  eggs  are  mature  these  spermatozoa  are  squeezed  out,  and 
fertilization  takes  place  on  the  outside  of  the  body. 

The  spermatozoa-forming  organs  are  more  complicated,  con- 
sisting essentially  of  three  pairs  of  closed  sacs,  called  the 

seminal  vesicles,  united  in  the  median  line  and  enclosing  the 
11 


156 


ORGANS  AND  ORGAN  SYSTEMS 


sperm  mother  cells  derived  from  the  testes.  Strictly  speaking, 
there  is  but  one  sac,  for  the  cavities  of  the  seminal  vesicles  are 
all  in  open  communication,  the  walls  of  the  sac  being  drawn  out 
in  three  pairs  of  lobes.  The  testes  are  small  and  difficult  to  find 
in  the  mature  worm,  for  the  dorsal  wall  of  the  vesicle  sac  must 
first  be  removed.  One  pair  are  situated  on  the  posterior  side 
of  the  anterior  wall  of  the  loth  somite,  and  another  pair  are 
in  the  corresponding  position  on  the  anterior  wall  of  the  nth 


v/// 


FIG.  63. — Diagram  of  the  reproductive  system  of  the  earthworm  showing  the 
central  chamber  of  the  seminal  vesicles  and  the  internal  position  of  the  testes. 

somite.  When  the  sperm  mother  cells  have  reached  a  certain 
stage  of  development,  they  drop  off  the  testis  and  continue  their 
development  as  free  cells  in  the  cavities  of  the  vesicles  (Figs. 
63  and  64). 

The  primordial  germ  cells  which  give  rise  to  the  spermatozoa  are  formed 
in  the  testes.  Here  the  nuclei  divide  without  cell  divisions,  until  multi- 
nucleated  protoplasmic  masses  are  formed,  which  break  loose  from  the 
testes  and  continue  their  development  in  the  seminal  vesicles  (Fig.  64). 


SPERM ATOGENESIS  OF  THE  EARTHWORM       157 

The  nuclei  increase  by  division  and  take  a  position  at  the  periphery  of  the 
protoplasmic  mass,  where  further  multiplication  follows  until  there  are 
from  32  to  64  nuclei.  Protoplasmic  furrows  then  cut  in  around  each  of 


FIG.  64. — Stages  in  the  development  of  spermatozoa  of  the  earthworm. 
(From  Calkins.) 

the  peripheral  nuclei.     Each  of  these  nuclei  then  begins  to  elongate,  and 
to  transform  into  a  spermatozoon.     The  nucleus  forms  the  head  of  the 


158  ORGANS  AND  ORGAN  SYSTEMS 

spermatozoon,  which  remains  attached  to  the  parent  protoplasmic  mass 
(blastophore) ;  the  centrosome  forms  the  middle  piece,  while  the  cyto- 
plasmic  tail  grows  out  at  the  distal  end.  The  bulk  of  the  spermatozoon 
thus  is  derived  from  the  nucleus. 

Under  the  dorsal  wall  of  the  median  vesicle  and  directly  op- 
posite each  testis  there  is  a  large  convoluted,  ciliated  opening  of 
the  sperm  duct.  These  ciliated  funnels  draw  the  mature 
spermatozoa  into  them,  and  thence  they  are  conducted  to  the 
outer  opening  of  the  ducts  on  the  i5th  somite.  The  ducts  from 
the  ciliated  funnels  on  each  side  of  the  worm  unite  to  form  a 
common  duct  leading  to  the  i5th,  so  that  two  common  sperm 
ducts,  known  as  the  vasa  deferentia,  open  on  the  ventral  surface 
(Fig.  63,  sperm  duct). 

Female  Organs  of  Reproduction. — These  are  much  simpler  in 
structure  than  the  male  organs,  consisting  of  one  pair  of  rela- 
tively large  ovaries  on  the  posterior  face  of  the  anterior  wall  of 
the  13  th  somite.  The  eggs,  when  mature,  drop  into  a  large- 
mouthed  thin-walled  funnel-like  oviduct  which  opens  on  the 
ventral  surface  of  the  i4th  somite  (Fig.  63). 

J.  REPRODUCTION.  FERTILIZATION  AND  DEVELOPMENT. — 
Fertilization  of  the  earthworm  eggs  takes  place  after  copulation, 
which  leaves  the  sperm  receptacles  of  the  worm  filled  with 
mature  spermatozoa.  A  tough  resistant  girdle  is  formed  around 
the  clitellum  of  each  worm,  and  after  the  worms  separate  this 
girdle  is  worked  forward,  collecting  albumen  from  the  glands 
on  the  ventral  surface,  mature  eggs  as  it  passes  the  i4th,  and 
mature  spermatozoa  as  it  passes  the  gth  and  loth  somites,  or 
openings  of  the  sperm  receptacles.  When  the  girdle  passes  off 
the  anterior  end,  it  closes  at  the  front  end  and  afterward  at  the 
posterior  end.  The  girdle  thus  forms  a  cocoon,  which  hardens 
later  into  a  chitinous  spindle-shaped  vessel  containing  repro- 
ductive cells  and  albuminoid  food  material  (Fig.  65).  The  eggs 
are  fertilized  in  the  coccoon  by  the  spermatozoa,  and  develop- 
ment begins  at  once,  continuing  under  the  protection  of  the 
cocoon. 

Cleavageof  the  egg  is  regular  up  to  the  i6-cell  stage,  with  four 
vegetative  cells  at  the  lower,  and  smaller  animal  cells  at  the 
upper  pole.  The  lower  cells  invaginate  and  form  a  typical  two- 


EMBRYOLOGY  OF  THE  EARTHWORM 


159 


layered  gastrula.  Up  to  this  stage,  development  closely  follows 
the  type  described  on  page  79,  but  from  here  on,  it  becomes 
complicated  by  the  formation  of  a  third  germ  layer,  called  the 
mesoderm.  This  arises  from  two  pole  cells,  coming  from  the 
vegetative  pole  and  taking  an  initial  position  in  the  segmenta- 
tion cavity  (Fig.  66).  They  then  divide,  forming  a  sheath  of 
cells  on  each  side  of  the  median  line  and  filling  the  segmentation 


FIG.  65. — A,  Egg  capsule,  enlarged  five  diameters  (a  few  eggs,  ov.,  are  shown 
near  by  on  the  right  enlarged  to  the  same  scale);  B,  an  ovum  highly  magnified; 
C,  a  spermatozoon  still  more  highly  magnified;  w,  nucleus  or  head;  m,  middle 
piece;  and  t,  tail.  (From  Sedgwick  and  Wilson.) 

cavity.  In  the  meantime,  the  embryo  has  elongated  in  the 
main  or  antero-posterior  axis  passing  through  the  blastopore; 
new  ectoderm  cells  are  pushed  in  from  the  ectoderm,  and  sec- 
ondary mesodermal  pole  cells  are  separated  from  the  mesoderm. 
The  former  are  the  first  stages  of  the  nervous  sytem,  and  are 
known  as  neuroblasts.  The  latter  are  of  different  kinds,  with 
different  functions  to  play  later.  Some  are  muscle-forming 
cells  called  somatoUask,  and  some  are  nephridia-forming, 
known  as  nephroblasts.  All  give  rise  to  sheets  of  cells  which  be- 
come differentiated  into  the  ultimate  adult  structures — nervous 
system,  muscles  and  nephridia. 

Meanwhile,  the  masses  of  mesoderm  cells  on  each  side  of  the 
median  line  begin  to  show  traces  of  a  loose  structure,  and  later, 
well-marked  spaces  or  cavities  are  developed  from  these  spaces 
and  assume  regular  shapes.  They  are  first  clearly  formed  in 
the  region  of  the  blastopore,  as  regularly  arranged  cavities  lined 
by  mesoderm  cells.  These  cavities  are  the  coelomic  cavities  of 
the  adult,  and  their  anterior  and  posterior  walls  form  the  dis- 


160 


ORGANS  AND  ORGAN  SYSTEMS 


en 


FIG.  66. — Diagrammatic  figures  of  the  early  and  embryonic  stages  in  develop- 
ment of  the  earthworm.  A-F,  gastrulation  showing  the  formation  of  the  two 
layers  ectoderm  and  endoderm,  and  the  beginning  of  the  mesoderm  arising  at  the 
outset  from  the  cells  shown  at  m;  the  lower  five  figures  show  the  origin  of  the 
coelomic  chambers  and  the  dissepiments,  al,  alimentary  tract;  an,  anus;  or, 
archenteron;  cce,  coelom;  ec,  ectoderm;  en,  endoderm;  m,  mesoblast  pole-cell; 
mz,  mesoblast  tissue;  mh,  mouth;  n,  nerve  cord;  sc,  segmentation  cavity;  sm, 
somatic  mesoderm;  spl.m,  splanchnic  mesoderm;  8,  eighth  coelomic  cavity. 
(From  Sedgwick  and  Wilson.) 


EMBRYOLOGY  OF  THE  EARTHWORM  161 

sepiments  marking  out  the  metameres  of  the  mature  animal. 
Thus  the  coelomic  cavities  are  mesodermal  in  origin  and  are 
lined  by  mesoblast,  this  lining,  known  as  endothelium,  thus 
having  a  different  origin  from  the  epithelium  of  the  gut  which 
comes  from  the  primary  endoderm,  or  from  the  epithelium  of 
the  skin  which  comes  from  the  primary  ectoderm.  The  primi- 
tive enteric  cavity  of  the  gastrula  gradually  develops  through  a 
series  of  changes  into  oesophagus,  crop,  gizzard,  and  stomach 
intestine.  The  ectoderm  turns  in  at  the  mouth  end  and  at  the 
posterior  end,  where  the  anus  breaks  through.  Two  regions  of 
the  digestive  tract  are  thus  lined  by  ectoderm,  that  of  the  mouth, 
termed  the  stomodaeum,  and  that  of  the  anal  end,  called  the 
proctodaeum.  The  chlorogogue  cells  are  formed  from  meso- 
derm,  as  are  also  the  blood  vessels,  muscles,  reproductive  organs 
and  seta  sacs.  The  young  worm  is  now  ready  for  an  indepen- 
dent life,  and  it  leaves  the  cocoon  after  from  two  to  three  weeks. 

SUMMARY  OF  THE  DERIVATIVES  OF  THE  GERM  LAYERS 

Endoderm.  Ectoderm.  Mesoderm. 

Oesophagus  Outer  epithelium          All  muscles 

Crop  Nervous  system  Endothelium  of  coelom 

Gizzard  Stomodaeum  Chlorogogue  cells 

Stomach-intestine         Proctodaeum  Calciferous  glands 

Ends  of  nephridia         Blood  vessels 
Dissepiments 

Nephridia,  functional  parts 
Seta  sacs 
Reproductive  organs 


CHAPTER  VII 
HOMOLOGY  AND  THE  BASIS  OF  CLASSIFICATION 

SYSTEMS  of  organs  similar  to  those  of  the  earthworm  are  found 
in  all  animals  higher  in  the  zoological  scale  than  Hydra  and 
the  coelenterates.  In  some  cases  the  organs  are  even  simpler 
than  those  of  the  earthworm,  but  in  the  great  majority  of  ani- 
mals they  are  more  complex.  The  complexity  is  brought  about 
by  the  specialization  of  parts,  leading  to  more  extensive  and 
more  detailed  division  of  labor.  The  power  of  modification 
possessed  by  animals  makes  possible  an  infinite  number  of  minor 
differences,  as  well  as  a  great  number  of  major  differences,  by 
which  we  mean  easily  recognizable  structural  differences.  A 
species  is  a  group  of  animals  or  plants  in  which  the  individuals 
differ  by  no  major  structural  differences.  It  is  estimated  that 
more  than  500,000  species  of  animals  exist  at  the  present  time. 

Species  of  like  nature  are  grouped  into  genera,  or  aggregates  of 
animal  types,  which  agree  in  the  main  elements  of  structure 
and  function.  Genera  in  turn  are  grouped  into  families, 
families  into  orders,  orders  into  classes,  and  classes  into  races  or 
phyla.  Different  phyla  have  few  characters  in  common; 
classes  have  numerous  common  features,  orders  still  more,  and 
so  on  down  to  species  in  which  all  characters  are  similar,  ex- 
cept for  minor  variations,  such  minor  features  giving  the  basis 
for  varieties. 

One  great  interest  to  biologists  in  the  study  of  comparative 
anatomy  is  to  trace  out  the  relationships  of  parts  which  have 
become  differentiated  from  generalized  organs.  Another  series 
of  problems  has  to  do  with  the  causes  which  have  led  to  such 
differentiation;  and  still  another  series  has  to  do  with  the  possi- 
bility of  inheritance  of  such  differentiations. 

Zoologists  recognize  some  seventeen  primary  phyla  or  races  of 
animals  as  follows: 

162 


CLASSIFICATION  OF  ANIMALS  163 

Group  comprising  Amoeba,  Euglena  Paramecium,  etc.,  upward  of  10,000    V 

species  Phylum  Protozoa 

Group  comprising  sponges  upward  of  800  species 

Phylum  Porifera. 
Group  comprising  Hydra,  sea-anemones,  etc.,  upward  of  3000  species 

Phylum  Coelenterata   */ 
Group  comprising  comb-bearing  jelly  forms,  upward  of  500  species 

Phylum  Ctenophora 
Group  comprising  tape  worm  and  flat  worms,  upward  of  1600  species 

Phylum  Platyhelminthes 
Group  comprising  round  worms,  filaria,  etc.,  upward  of  1000  species 

Phylum  Nemathelminthes 
Group  comprising  ringed  worms,  earthworm  types,  upward  of  2500  species  * 

Phylum  Annelida 
Group  comprising  lobster,  crab,  shrimp  and  allies,  upward  of  8000  species 

Phylum  Crustacea 
Group  comprising  insects  breathing  by  tracheae,  upward  of  300,000  species 

Phylum  Insecta 
Group  comprising  centipedes,  spiders,  ticks,  etc.,  upward  of  5000  species 

Phylum  Arachnida 
Group  comprising  clams,  snails  and  allies,  upward  of  22,000  species 

Phylum  Mollusca 
Group  comprising  star  fish,  sea  cucumbers,  etc.,  upward  of  2500  species 

Phylum  Echinodermata 

Group  comprising  fish,  frogs,  reptiles,  birds,  mammals,  upward  of  25,000 
species.  Phylum  Vertebrata 

In  addition  to  these  there  are  several  minor  races  which  are 
recognized  by  biologists,  but  in  which  the  number  of  species  is 
comparatively  small;  here,  for  example,  are  the  phyla  Rotifera 
(350  species),  Polyzoa  (700  species),  Brachiopoda  (100  species) 
and  Tunica ta  (300  species). 

No  one  has  made  an  accurate  enumeration  of  the  existing 
species  of  animals,  but  it  is  safe  to  say  that  more  than  350,000 
species  are  known  and  grouped  into  distinct  phyla. 

In  each  phylum,  although  the  type  is  the  same  throughout, 
the  structures  may  be  so  modified  as  to  give  very  distinct  forms 
of  animals.  Different  types  of  vertebrates  give  the  most  fa- 
miliar examples  of  this,  mammals,  birds,  reptiles,  amphibia  and 
fish  being  widely  different  from  one  another,  yet  all  belong  to  the 
same  phylum.  Birds,  being  fundamentally  of  one  type,  are 


164 


HOMOLOGY 


grouped  together  as  a  class;  mammals,  reptiles,  etc.,  form  other 
and  fairly  homogeneous  classes,  five  classes  in  all,  in  the  race  of 
vertebrates. 

Further  subdivision  is  necessary  for  the  complete  classifica- 
tion of  animals.  The  classes  which  form  the  most  comprehen- 
sive groups  within  the  phyla  are  frequently  broken  up  into  sub- 
classes, and  these  into  orders,  the  basis  of  classification  being 
structures  or  mode  of  life,  or  some  other  pronounced  characteris- 
tic or  aggregate  of  characteristics.  The  sub-class  Oligochaeta, 
for  example,  includes  a  group  of  worms  inhabiting  fresh  water, 
and  another  group  which  burrow  into  the  earth.  The  former 
are  classified  as  an  Order  Limicola, while  the  latter  are  placed  in 
the  Order  Terricola.  Orders,  in  turn,  are  sub-divided  into  sub- 
orders and  families,  and  the  families  into  genera,  the  basis  of 
classification,  as  before,  being  structures  where  possible,  or  some 
prominent  characteristic.  Thus  Megascolex,  Allolobophora, 
etc.,  are  similar  to  Lumbricus,  the  earthworm,  forming  different 
genera  in  the  common  family,  Lumbricidae.  According  to 
such  a  scheme,  therefore,  the  animals  studied  here  are  classified 
as  follows: 

GENUS          SPECIES        FAMILY          ORDER        CLASS        PHYLUM 
Amoeba  proteus        Gymnam-     Rhizopoda    Sarcodina    Protozoa 

oebidae 
Euglena  viridis         Euglenidae  Euglenida     Mastigo-     Protozoa 

phora 
Paramecium     caudatum   Parame-      Holotrichida  Infusoria     Protozoa 

cidae 

Hydra  fusca,          Hydridae      Leptolina    Hydrozoa      Coelen- 

viridis  terata 

Taenia  solium         Taeniidae     Polyzoa       Cestoda        Platyhel- 

minthes 
Lumbricus        terrestris     Lumbri-        Oligochae-  Chaetopoda  Annelida 

cidae  tida 

Homarus  Americana  Astacidae      Decapoda    Malacos-       Crustacea 

traca 

Callinectes        hastatus      Cancridae     Decapoda   Malacos-       Crustacea 

traca 

At  first  glance  it  is  often  difficult  to  classify  animals  even  to 
the  phylum,  and  in  some  cases  only  a  prolonged  study  furnishes 


ANALOGY  AND  HOMOLOGY  165 

the  key  to  relationships.  Animals  that  fly,  for  example,  includ- 
ing bats,  birds,  and  insects,  all  have  wings,  and  might  be  classi- 
fied in  one  group  as  "  beasts  of  the  air."  But  study  of  bats  and 
birds  shows  that  they  belong  to  two  entirely  different  classes,  the 
bats  having  wings  like  the  arms  and  fingers  of  a  mammal  and 
the  mammary  glands  of  the  mammals,  while  birds  have  espe- 
cially modified  fore  limbs,  entirely  different  bone  structure  and 
other  organs,  which  place  them  in  the  class  Aves.  Birds  and 
insects  are  also  different,  both  in  the  character  of  the  wings 
and  in  the  absence  of  an  internal  bony  skeleton  in  the  latter. 
While  the  functions  of  wings  of  birds  and  insects  are  the  same, 
their  anatomy  shows  an  entirely  different  mode  of  origin  and 
different  secondary  structures.  In  such  cases  the  organs  are  said 
to  be  analogous.  When  organs  have  the  same  ancestry,  that  is, 
when  they  come  from  some  common  part  of  an  ancestral  type, 
they  are  said  to  be  homologous.  The  wings  of  a  bird  have  had 
the  same  ancestry  as  the  fore-legs  of  beasts  or  the  arms  of  man; 
so  too  have  the  wings  of  a  bat — hence  arms,  fore-legs  of  beasts, 
and  wings  of  bat  or  bird  are  homologous  structures.  It  is  quite 
otherwise  with  the  wings  of  a  bee  or  fly.  These  have  had  an 
entirely  different  ancestry  from  the  wings  of  a  bird  and  are 
not  homologous  with  the  latter.  Wings  of  different  insects, 
however,  are  homologous. 

Homology,  or  genetic  relationship  of  organs  and  structures 
in  general,  is  the  ground  principle  of  classification  of  species. 
Two  organs  on  different  animals  may  be  homologous  whether 
they  perform  the  same  functions  or  not,  and  conversely,  the 
same  functions  may  be  performed  by  organs  not  homologous. 
The  study  of  homologies  therefore  is  one  of  the  most  important 
in  comparative  anatomy  and  in  taxonomy.  The  walking  legs 
of  vertebrates  and  those  of  a  lobster  are  the  same  in  function 
and  are  analogous  organs,  but  no  one  would  compare  them 
morphologically,  and  they  are  not  homologous  in  any  sense.  It 
is  quite  otherwise,  however,  with  the  legs  of  lobsters,  of  crabs 
and  of  shrimps,  which  are  homologous,  having  had  a  like  origin. 
All  of  the  appendages  of  a  lobster  or  crab,  furthermore,  although 
they  have  widely  different  functions  and  are  quite  different  in 


166  HOMOLOGY 

form  and  appearance,  are  serially  homologous  with  one  another. 
The  Crustacea  therefore  give  excellent  subject  matter  for  the 
study  of  homology. 

I.  THE  AMERICAN  LOBSTER,  HOMARUS  AMERICANUS 

HABITS,  MODE  or  LIFE. — Lobsters  live  in  comparatively 
shallow  waters  along  the  Atlantic  coast  from  Labrador  to  Dela- 
ware, in  depths  of  from  one  to  100  fathoms.  They  are  preda- 
tory, but  usually  capture  their  prey  by  stealth,  while  hidden  in 
weeds  on  the  sea  bottom.  They  are  also  well-known  scavengers, 
and  will  quickly  discover  and  devour  dead  fish  to  which  they  are 
attracted  through  an  acute  sense  of  smell.  Ungainly  on  land, 
their  movements  in  water  are  graceful,  where  they  may  run 
about  -with  agility  or  shoot  backward  with  surprising  speed. 
When  enemies  are  about  they  are  pugnacious,  but  at  the  same 
time  wary  and  resourceful,  and  are  well  able  to  defend  them- 
selves. A  closely  related  species  is  the  European  lobster 
(Homarus  gammarus),  while  somewhat  similar  forms  are  the 
langouste  of  the  French  coast,  and  the  so-called  Norwegian 
lobster  (Nephrops  norvegicus). 

GENERAL  STRUCTURE  AND  SYSTEMS  OF  ORGANS. — Like  the 
earthworm,  the  lobster  and  all  of  its  allies  are  metameric  ani- 
mals. The  somites  or  metameres  may  easily  be  seen  in  the 
abdomen,  where  they  are  separate,  but  in  the  anterior  region 
they  are  fused  together,  those  of  the  head  (cephalon)  fusing 
with  those  of  the  thorax  to  form  the  main  part  of  the  animal, 
termed  the  cephalothorax  (Fig.  67).  All  parts  of  the  body  are 
covered  by  a  firm  lifeless  cuticle  of  chitin  which,  on  the  back 
(dorsum)  and  on  the  side  (tergum),  is  impregnated  with  calcium 
salts,  until  quite  solid.  In  some  species  this  covering  becomes 
almost  rock-like  in  its  solidity,  containing  much  pure  limestone. 
The  chitin  and  lime  are  secreted  by  cells  of  the  skin,  which  is 
drawn  down  over  the  sides  of  the  cephalothorax  in  two  great 
folds  like  the  front  flaps  of  a  coat;  the  two  flaps  thus  cover 
and  protect  two  branchial  chambers  on  the  two  sides  of  the  cepha- 
lothorax, where  the  gills  lie,  and  are  called  the  branchiostegites 


EXTERNAL  STRUCTURES  OF  THE  LOBSTER      167 

or  gill  protectors.  On  the  under  side  of  the  body,  especially  in 
the  abdomen,  the  chitin  is  thin  and  transparent,  with  heavier 
ribs  of  chitin  for  muscular  support,  while,  in  the  region  of  the 
cephalothorax,  these  heavier  bars  are  united  to  form  an  internal 
skeleton-like  structure,  termed  the  endophragmal  skeleton. 
This  forms  the  floor  of  the  body  cavity,  and  protects  the  ven- 
tral chain  of  ganglia  (Fig.  69,  p.  171). 

APPENDAGES  AND  SERIAL  HOMOLOGY. — The  metameric  struc- 
ture of  the  body  is  well  indicated  by  the  appendages,  of  which 
there  is  one  pair  to  each  somite.  The  relation  of  the  appendages 
to  the  somite  is  clearly  shown  in  the  abdominal  region  where  the 


FIG.  67. — The  American  lobster,  Homarus  americanus,  showing  regions  of  the 

body. 

somites  are  free,  but  in  the  cephalothorax  where  the  somites  are 
fused,  their  external  signs  are  given  only  by  the  several  pairs  of 
appendages  lying  closely  packed  over  one  another.  Some  of 
these  belong  to  the  head,  and  some  to  the  thorax.  The  number 
of  somites  in  lobsters  and  in  all  of  the  higher  types  of  Crustacea 
is  limited  to  20,  of  which  5  form  the  head,  8  the  thorax,  and  6  the 
abdomen  (some  zoologists  allow  6  for  the  head) .  Correspond- 
ing to  these  somites,  there  are  19  (or  20)  pairs  of  appendages 
which  are  named  according  to  their  functions,  and  are  distrib- 
uted as  follows: 


168 


HOMOLOGY 


Antennae 
Antennules 
Head  {  Mandibles  or  jaws 
ist  maxillae 
2nd  maxillae 


Thorax 


Abdomen 


ist  Maxillipedes 
2nd  Maxillipedes 
3rd  Maxillipedes 
ist  Ambulatory  (chelae) 
2nd  Ambulatory 
3rd  Ambulatory 
4th  Ambulatory 
^  5th  Ambulatory 

Copulatory 

ist  swimmerets 

2nd  swimmerets 

3rd  swimmerets 

4th  swimmerets 

5th  swimmerets,  or 
caudal  appendages 

Telson  (unpaired) 

The  terminal  joint  of  the  abdomen,  termed  the  telson,  bears 
the  anus,  and  is  not  usually  regarded  as  a  somite. 

Different  as  they  are  in  function  and  different  as  they  appear  to 
be  in  structure,  the  appendages  are  all  built  upon  the  same  plan, 
and  throughout  the  series  we  can  trace  the  same  homologous 
parts.  The  simplest  of  all  are  the  appendages  of  the  abdomen, 
where  three  fundamental  parts  can  be  easily  distinguished,  a 
basal  portion,  termed  the  protopodite,  attached  to  the  body,  and 
two  distal  portions,  one  of  which  is  inside,  that  is  near  the 
median  line  of  the  animal,  the  other  outside.  The  internal  part 
is  called  the  endopodite,  the  external  part  the  exopodite  (Fig.  68) . 
In  the  male,  the  first  abdominal  appendages  show  considerable 
modification  from  the  others.  Here  the  external  parts  have 
disappeared,  leaving  only  the  endopodites  which  are  tightly 
fused  with  the  protopodites  to  form  the  copulatory  organ.  In 
the  female,  the  appendages  of  this  somite  are  degenerated,  as 
shown  by  the  entire  absence  of  distal  parts,  leaving  only  the 
protopodites  which  are  drawn  out  into  plume-like  organs.  The 
terminal  appendages  are  similar  to  the  other  abdominal  append- 
ages, but  are  much  enlarged  in  all  parts  and  strengthened  by 
chitin  and  lime  salts. 

The  thoracic  appendages  are  highly  modified.  All  ten  of  the 
ambulatory  consist  of  one  distal  branch  only,  the  endopodite, 


APPENDAGES  OF  THE  LOBSTER 


169 


FIG.  68.— The  appendages  from  the  entire  right  side  of  the  body  of  a  lobster, 
arranged  serially  to  illustrate  serial  homology. 


170  HOMOLOGY 

attached  to  the  protopodites  which,  in  turn,  are  freely  attached 
to  the  body.  On  the  fifth  ambulatory  protopodites  and  on  the 
internal  surfaces,  are  the  openings  of  the  male  organs  of  repro- 
duction (Fig.  68,  14).  Here  also  the  skin  or  membrane  of  the 
protopodite  is  drawn  out  into  a  leaf-like  organ,  termed  the  bract 
or  flabellum,  a  structure  which  reappears  in  each  of  the  thoracic 
appendages  and  serves  as  a  partition  wall  between  the  gills  in 
the  branchial  chamber.  All  of  the  other  ambulatory  append- 
ages are  like  the  fifth  in  consisting  of  one  shaft,  the  endopodite, 
but  on  the  protopodites  of  the  first  four,  in  addition  to  the  bracts, 
there  are  outgrowths  of  membrane  which  form  the  gills  in  the 
branchial  chamber  (Fig.  68,  gill).  The  endopodites  are  jointed, 
consisting  of  five  parts  or  joints  termed  podomeres.  There  is 
nothing  in  their  structure  to  show  that  they  are  endopodites  and 
not  exopodites,  this  fact  being  established  by  embryology,  all  of 
the  thoracic  limbs  appearing  first  as  biramous  appendages  with 
both  exopodites  and  endopodites  (see  Fig.  78).  The  exopodites 
wither  and  disappear  as  growth  progresses,  leaving  only  the  en- 
dopodites. Similarly  with  the  antennae,  jaws  and  antennules, 
the  exopodites  have  disappeared  or  are  so  highly  modified  as  to 
be  indistinguishable,  leaving  only  the  inner  branches.  The  re- 
maining appendages  of  head  and  thorax  are  not  so  highly  modi- 
fied that  homologous  parts  cannot  be  made  out,  although  they 
must  be  studied  part  by  part  with  the  principles  of  homology  in 
mind.  These  parts  are  well  shown  in  the  accompanying 
figures. 

All  of  these  diverse  appendages  have  been  developed  from  the 
primitive  simple  type  of  the  biramous  appendage  of  the  abdo- 
men, and  well  illustrate  the  principle  of  adaptation  for  particular 
functions.  The  walking  legs,  for  example,  are  adapted  for  this 
means  of  locomotion,  and  the  anterior  pair  for  offence  and  de- 
fence ;  the  jaws  for  crushing  food ;  the  maxillae  and  maxillipedes 
for  seizing,  sifting  and  propelling  food  into  the  jaws.  It  would 
seem  as  if  unnecessary  parts  of  the  appendages  had  disappeared, 
leaving  only  those  portions  which  are  useful  for  the  purpose  of 
the  particular  appendage.  Many  biologists  hold  that  such 
adaptations  come  through  use  or  disuse  of  parts,  the  useless  por- 


INTERNAL  STRUCTURES  OF  THE  LOBSTER       171 


12 


172  HOMOLOGY 

tions  degenerating,  the  useful  parts  increasing  in  usefulness  by 
continued  activity.  Another  group  of  biologists,  however,  take 
the  very  opposite  view,  viz.,  that  the  function  or  use  of  an  organ 
depends  upon  its  position,  the  maxillae  and  maxillipedes,  for 
example,  crowded  together  in  the  thorax,  have  the  functions  of 
seizing,  sifting  and  propelling  food  matters  forced  upon  them, 
and  could  not  do  otherwise.  In  either  case,  there  is  general 
agreement  that  all  appendages  are  derived  from  one  ancestral 
biramous  type  of  appendage,  which  is  regarded  as  a  general- 
ized organ  capable  of  differentiation  and  development  along 
different  lines,  until  structures  result  of  widely  different  appear- 
ance, although  homologous  throughout.  The  study  of  com- 
parative anatomy  is,  in  large  part,  only  the  ferreting  out  of  such 
homologies  in  animals  of  the  same  or  allied  groups  (see  Chapter 
IX). 

DIGESTIVE  SYSTEM. — The  lobster  is  primarily  a  scavenger, 
and  eats  all  forms  of  dead  and  decaying  protein  matter.  For 
this  purpose,  it  has  a  highly  developed  digestive  apparatus, 
capable  of  extracting  the  nutrient  material  out  of  all  sorts  of 
food. 

The  most  conspicuous  part  of  the  digestive  system  is  the 
chitinous  fore-stomach  or  cardiac  stomach  into  which  the 
oesophagus  opens  (Fig.  69) .  In  the  walls  of  this  organ  special 
chitinous  processes  are  developed,  forming  tooth-like  accumula- 
tions which  are  worked  by  special  muscles  attached  to  the  body 
wall.  These  teeth  form  a  grinding  machine,  known  as  the 
gastric  mill,  which  triturates  the  food  passed  on  by  the  jaws. 
They  also  form  a  sieve,  guarding  the  opening  into  the  functional 
or  pyloric  stomach  and  preventing  all  bones  or  large  materials 
from  entering  the  physiological  stomach,  in  which  digestive 
juices  are  poured  from  the  large  digestive  glands  known  as  the 
hepato-pancreas.  After  action  by  these  fluids,  the  undigested 
residue  is  passed  on  to  the  intestine  which  lies  over  the  dorsal 
sides  of  the  ventral  muscles. 

Not  only  are  digestive  fluids  poured  into  the  pyloric  stomach, 
but  some  of  it  also  goes  into  the  cardiac  stomach,  where  the 
food  particles  are  softened  and  prepared  for  passing  the  gastric 


DIGESTIVE  SYSTEM  OF  THE  LOBSTER  173 

filter  between  the  cardiac  and  pyloric  portions.  Food  thus 
passed  through  is  distributed  in  the  various  diverticula  of  the 
hepato-pancreas,  where  the  bulk  of  digestion  takes  place.  Here 
also  are  the  absorbing  cells  which  take  up  the  digested  foods  and 
turn  them  over  to  the  blood  (Jordan).  The  end  gut  or  intes- 
tine plays  no  role  either  in  digestion  or  in  absorption.  The 
absorption  cells  have  the  same  general  structure  as  those  of  the 
earthworm.  The  connective  tissue  in  which  the  hepato-pan- 
creas is  embedded  is  richly  supplied  with  blood  vessels  and 
lymph  spaces,  which  probably  receive  digested  food  directly 
from  the  absorption  cells. 

The  digestive  fluid  which  comes  from  the  hepato-pancreas  is 
very  complex.  It  is  of  yellowish-brown  color,  not  viscous,  is 
rich  in  albumen,  contains  a  free  alkali,  and  gives  a  flocculent 
precipitate  with  acids.  This  precipitate,  filtered  and  washed, 
gives  all  of  the  reactions  of  a  globulin.  The  digestive  ferments 
contained  in  this  juice  are  (i)  a  protease  or  proteolytic  ferment 
similar  to  the  protein  digestive  ferments  of  the  earthworm  and 
other  invertebrates;  (2)  a  lipase  or  fat  emulsifying  ferment;  (3) 
an  amylase  or  starch  converting  ferment.  In  other  allied 
forms  of  Crustacea,  still  more  ferments  have  been  obtained  from 
the  digestive  juices,  and  these  may  be  present  in  the  lobster. 
Thus  a  cellulose  dissolving  ferment  (cytase)  was  discovered  by 
Biedermann  and  Moritz  from  the  crayfish. 

The  digestive  tract  of  the  lobster  thus  shows  a  considerable 
advance  over  that  of  the  earthworm  or  other  lower  types.  The 
functional  digestive  part  is  removed  from  the  main  tract,  but  is 
derived  from  it  as  an  outgrowth  or  diver ticulum.  It  represents 
a  step  toward  still  higher  types  of  development,  where  secretions 
from  different  glands  are  poured  into  a  digestive  sac  or  stomach 
and  intestine.  Here  in  the  lobster,  the  digestive  gland  still  acts 
as  a  part  of  the  glandular  tube  of  a  worm;  the  food  is  contained 
in  it,  and  digestion  and  absorption  take  place  in  it  instead  of  in 
the  main  digestive  tube.  In  higher  animals,  all  of  the  glands 
pour  their  digestive  fluids  into  the  main  tube. 

The  Blood  Vascular  System. — In  the  lobster  and  other  forms 
of  arthropods,  all  of  the  blood  of  the  organism  passes  sooner  or 


174  HOMOLOGY 

later  from  the  main  arteries  into  the  general  cavity  of  the  body, 
where  the  food  material  is  taken  up.  Such  a  circulation  of 
blood  is  spoken  of  as  an  open  circulation,  as  opposed  to  the 
closed  circulation  of  organisms  like  the  earthworm,  which  have 
both  arterial  and  venous  capillaries  so  that  the  blood  is  always 
within  specialized  blood  vessels.  The  body  cavity  of  the  lob- 
ster, therefore,  is  quite  different  from  the  coelom  of  an  earth- 
worm and  other  animals.  It  is  not  lined  by  endothelium,  and 


branchial 

chamber 


haemocoel^. 


FIG.  70. — Transverse  section  through  the  thorax  of  a  lobster  to  show  the  rela- 
tion of  the  gills  to  the  bronchial  chamber,  the  haemocoel,  and  the  chamber  of  the 
heart.  (Modified  after  Lang.) 

does  not  contain  the  opening  of  the  excretory  organs  (nephridia), 
nor  do  its  walls  give  rise  to  the  germ  glands.  It  corresponds 
rather  to  a  large  blood  sinus,  and  for  this  reason  is  termed  a 
haemocoel  and  not  a  coelom.  The  real  coelom  of  these  forms  is 
limited  to  the  small  cavities  of  the  nephridia  and  the  germ  glands. 
Gills. — The  blood  mixed  with  digested  food,  in  the  haemocoel, 
passes  slowly  into  the  gills,  where  it  is  aerated.  The  gills  are 
pockets  of  tissue  derived  from  the  epithelium,  drawn  out  in  the 
form  of  long  triangular  pyramids  with  broad  bases  and  pointed 


VASCULAR  SYSTEM  OF  THE  LOBSTER  175 

tips  (Fig.  70) .  Each  gill,  in  turn,  is  drawn  out  into  innumerable 
flaps  or  lamellae,  closely  packed  together  like  leaves  of  a  book. 
In  each  gill  there  are  two  blood  vessels,  one  ventral,  one  dorsal. 
The  blood  from  the  body  cavity  enters  the  ventral  vessel,  pass- 
ing into  the  gills;  here  capillary  vessels  branch  into  the  lamellae, 
and  are  continuous  with  similar  capillaries  emptying  into  the 
dorsal  vessel.  In  the  gills,  therefore,  there  is  a  small  closed 
circulation  from  one  venous  ventral  tube  into  another  dorsal 
tube,  in  which  it  passes  toward  the  heart.  The  thin-walled 
lamellae  of  the  gills  are  in  contact  with  water,  which  passes 
through  the  branchial  chamber  by  activity  of  the  scoop  or 
scaphognathite,  which  consists  of  the  fused  bract  and  exopodite 
of  the  second  maxilla  (Fig.  68).  The  blood  is  thus  brought  in 
contact  with  fresh  water  and  is  aerated,  giving  off  CO2,  and 
taking  oxygen  before  passing  to  the  dorsal  branchial  tube. 

Various  parts  of  the  body  wall  are  drawn  out  to  form  these 
triangular  gill  pockets.  Some  are  on  the  appendages  and  are 
termed  podobranchs;  others  are  on  the  basal  joint  of  the  append- 
age and  do  not  come  out  when  the  appendages  are  removed. 
These  are  termed  arthrobranchSj  from  their  position  on  the 
joints;  still  others  originate  on  the  body  wall  itself,  and  are 
termed  pleurobranchs  or  side-wall  gills.  The  number  of  each 
kind  gives  the  basis  for  a  gill  formula,  which  differs  with  each 
species  of  Crustacea ;  the  formulae  for  the  lobster  and  the  cray- 
fish are  given  below: 

Podobranchiae  Arthrobranchiae  Pleurobranchiae 

Crayfish  (Astacus).  ..  .6  n  3  (2  rudimentary) 

Lcbster  (Homarus)  .  .  .6  10  4 

After  aeration  in  the  gills,  the  blood  is  slowly  passed  on  into 
large  branchial  sinuses  (branchio-cardiac  sinuses),  which  lead 
into  the  pericardial  chamber  containing  the  heart.  The  latter 
has  the  form  of  a  pentagonal  shield  with  six  openings  or  ostia, 
of  which  one  pair  is  dorsal,  one  lateral,  and  one  ventral.  Blood 
from  the  pericardium  enters  the  heart  through  these  ostia  and 
is  prevented  from  going  back  again  by  valves  on  the  inside, 
which  are  closed  upon  pressure  due  to  contraction  of  the  heart. 
This  pressure  forces  the  blood  out  into  arteries  as  follows: 


176 


HOMOLOGY 


one  unpaired  median  artery,  ophthalmic,  which  conveys  blood 
to  the  eyes  and  surrounding  organs;  one  pair  of  antennary 
arteries  from  the  anterior  sides  of  the  heart,  leading  to  the 
antennae  and  adjacent  organs;  one  pair  of  hepatic  arteries  from 
the  lateral  ventral  part  of  the  heart,  which  are  quickly  lost  in 


white 
portion— 


cortical 
-portion 


cortical  portion 


white,  portion 


FIG.  71. — Diagram  of  the  kidney  of  a  crayfish   (Astacus  flumatilis) . 
Parker  and  Haswell  after  Marchal.) 


(From 


branches  in  the  hepato-pancreas ;  one  unpaired  dorsal  abdominal 
artery  from  the  posterior  angle  of  the  heart,  and  one  unpaired 
sternal  artery,  which  passes  directly  downward  from  the 
posterior  ventral  portion  of  the  heart  to  the  ventral  surface, 
where  it  enters  the  ventral  artery  which  traverses  the  entire 
ventral  surface.  The  dorsal  abdominal  continues  posteriorly 


EXCRETORY  SYSTEM  OF  THE  LOBSTER        177 


to  the  telson,  giving  off  one  pair  of  large  arteries  in  each  somite 
(Fig.  69). 

The  vascular  system  thus  consists  of  an  arterial  system  and  a 
great  body  cavity,  which  forms  a  blood  sinus,  taking  the  place  of 
a  venous  system  in  other  ani- 
mals. The  pressure  forcing  the 
blood  through  the  gills  comes 
from  the  constant  addition  of 
blood  to  the  body  cavity  through 
muscular  heart  beats,  aided  by  the 
vacuum  produced  when  the  heart 
is  emptied.  Movements  of  the 
appendages  also  tend  to  keep  up 
a  constant  circulation  in  the 
sinuses. 

THE  EXCRETORY  SYSTEM. — Ex- 
cretion in  the  lobster  must  be 
comparatively  sluggish,  for  the 
organs  for  the  purpose  are  small 
and  poorly  placed  for  active  func- 
tion. This  may  be  due  to  the  fact 
that  the  lobster  and  similar  forms 
are  naturally  sluggish  animals, 
lying  in  wait  for  prey,  feeding  on 
carrion,  etc.,  rather  than  moving 
about  actively  in  search  of  food. 
The  nephridia  are  small  flattened 
coiled  organs  at  the  bases  of  the 
antennules,  and  consist  of  a  rather 
large  "bladder"  and  a  small 
glandular  part  (Fig.  71).  From 
their  characteristic  color  they  are 
also  known  as  the  green  glands. 


FIG.  72. — The  abdominal 
musculature  of  the  lobster  to 
show  the  complicated  arrange- 
ment of  extensors  and  flexors. 
(From  Gerstaecker,  after  Milne- 
Edwards.) 


The  external  openings  of  the  nephridial  ducts  are  on  the 
inner  faces  of  the  basal  segments  (Fig.  68,  3).  Some  excretion 
of  waste  matters  may  also  take  place  through  the  skin. 

THE  MUSCULAR  SYSTEM. — The  muscles  of  the  lobster  are 


178 


HOMOLOGY 


FIG.  73. — The  central  nervous  systems  of  the  lob- 
ster and  the  crab.  The  ventral  chain  of  ganglia  in 
the  crab  are  concentrated  in  one  ventral  mass,  the 
sternal  artery  passing  through  it.  (From  Gerstaecker 
after  Cuvier.) 


highly  developed,  the  large  ventral  muscles 
of  the  abdomen  being  remarkably  power- 
ful. These  are  inserted  anteriorly  on 
the  inner  walls  of  the  cephalothorax  (Fig. 
72),  one  on  each  side.  In  the  abdomen 
they  twist  around  one  another  like  a 
huge  muscular  rope,  and  are  intimately 
connected  with  the  ventral  exo-skeletal 
parts  of  each  somite.  Contraction  of  the 
muscles  results  in  the  simultaneous  ven- 
tral turning  of  all  the  abdominal  somites, 
and  the  vigorous  flop  of  the  lobster 
results.  Similar,  but  smaller  and  straight 
muscles  lie  on  the  dorsal  surface  of  the 
huge  ventral  muscles,  and  are  similarly 
connected  with  the  dorsal  exo-skeleton. 
When  these  muscles  contract,  the  abdomi- 


NERVOUS  SYSTEM  OF  THE  LOBSTER 


179 


nal  segments  are  straightened  out.  These  flexor  and  extensor 
muscles  thus  act  quite  differently  from  the  dermal  musculature  of 
the  earthworm.  Other  important  muscles  work  the  various  ap- 
pendages, of  which  those  of  the  giant  chelae  are  the  most  highly 
developed.  Still  others  manipulate  the  gastric  mill,  theeyes,  etc. 
THE  NERVOUS  SYSTEM. — In  general  arrangement,  the  nervous 
system  of  the  lobster  is  strik- 
ingly similar  to  that  of  the 
worm;  here  again  we  find  a 
ventral  chain  of  nerve  gan- 
glia which,  however,  are  dor- 
sal to  the  ventral  blood  vessel. 
A  pair  of  cerebral  ganglia, 
close  to  the  eyes,  innervates 
these  and  adjacent  organs. 
A  long  pair  of  circumoe- 
sophageal  commissures  con- 
nects the  cerebral  with  the 
first  ventral  or  sub-cesoph- 
ageal  ganglia.  These,  how- 
ever, represent  a  fusion 
of  thoracic  ganglia,  for  just 
as  the  somites  here  have 
merged  to  form  the  cephalo- 
thorax,  so  these  ganglia  have 
fused  into  one.  Between 
the  fourth  and  fifth  ganglia,  FIG  M  otocyst  o{  the 

the   double    nerve   Cord   splits  opened,  showing  sensory  hairs  and  otoliths. 

and  allows  the  sternal  artery  (From  G«sta«k«aft«  Farre-> 
to  pass  through.     In  the  abdomen,  the  nerve  chain  is  quite 
regular  and  similar  to  that  of  the  earthworm,  in  having  one 
pair  of  ganglia  to  each  somite  (Fig.  73). 

SENSE  ORGANS. — In  the  earthworm,  we  have  seen  that  there 
is  a  well-marked  advance  in  nerve-organization  over  forms  like 
Hydra,  with  grounds  for  dividing  it  into  peripheral  sensory  and 
internal  central  nervous  systems.  The  peripheral  system  con- 
sists of  more  or  less  isolated  sensory  cells  with  their  nerve 


180 


HOMOLOGY 


distal 
netinula 

cells... 


processes,  more  plentiful  about  the  anterior  end,  but  distributed 
nevertheless  about  the  entire  body. 

In  arthropods  we  find  a  great  advance,  over  annelids,  in  com- 
plexity of  the  peripheral,  or  sensory,  nervous  system.  Here 
similar  sensory  cells  are  grouped  together  to  form  different  kinds 

of  sensory  organs  of  more  or 
less  complexity.  In  the  lob- 
sters we  recognize:  (i)  tactile 
organs;  (2)  olfactory  or  smell- 
ing organs;  (3)  auditory  or 
primitive  hearing  organs,  and 
(4)  organs  of  vision  or  eyes. 

1.  Tactile       Organs. — The 
organs  of  "touch  are  distrib- 
uted over  the  body,  usually 
on   the    appendages   and  in 
large  numbers  in  the  cephalic 
region,  in  the  form  of  hairs. 
Each  hair  contains  a  nerve, 
with  delicate  nerve  endings 
in  cells  forming  the  walls  of  the 
hair,  and  each  hair  contains 
a  small  ganglion. 

2.  The  olfactory  organs  are 
similar  to  the  tactile,  but  dif- 
fer in  the  position  of  ganglia 
and     arrangement     of     the 
nerve    endings.     They    are 
distributed    mainly    on    the 
antennules. 

3.  The  auditory  organs  are 
technically     termed      "oto- 

FIG.  75.— Three  ommatidia  from  the    cysts,"    and    their    functions 
compound  eye  of  the  lobster.     (Modi-  .,     -,    ,1  i_    4.1, 

fied  after  Parker.)  are  incited  through  the  ac- 

tion    of      small      crystalline 

foreign  bodies,  termed  "otoliths."     The  cysts  or  capsules  are 
located  on  the  inner  side  of  the  basal  joints  of  the  antennules, 


+... distal   .  .      '• 
I     retinula  cells 


A,... cone  cell 


Ivy     '  ntlnuta:  cells 

•II, 

fl_ rhabdotne 


fibre 


SENSORY  ORGANS  OF  THE  LOBSTER  181 

and  open  to  the  outside  by  a  hair-protected  aperture  at  the 
distal  angle  of  the  membranous  wall  of  the  capsule  (Fig. 
74).  Inside  the  capsule  is  a  gelatinous  mass  of  semi-fluid 
material,  through  which  fine  sensory  hairs,  innervated  from  a 
main  sensory  branch,  are  abundantly  distributed.  The  otoliths 
or  crystals  are  also  distributed  throughout  the  gelatinous 
matrix.  When  the  equilibrium  of  the  organism  is  disturbed 
these  otoliths  impinge  on  the  sensory  hairs,  and  thus  originate 
stimuli  and  motor  responses  by  which  the  animal  regains  its  bal- 
ance. Sound  vibrations  may  also  have  the  same  effect  on  the 
hairs  directly.  The  so-called  "  auditory  "  organ  perhaps  has  less 
to  do  with  sound  vibrations  than  with  the  balance  or  equilibrium 


FIG.  76. — Centrolecithal  type  of  egg  and  cleavage  in  the  crayfish.  The  nuclei, 
after  several  divisions,  pass  to  the  periphery  of  the  egg  after  which  radial  cleavage 
planes  divide  it  into  cells.  (From  Parker  and  Haswell.) 

of  the  body,  and  compares  with  the  lateral  organs  of  fishes  or 
the  semi-circular  canals  of  vertebrates. 

4.  The  Eyes. — The  eyes  of  arthropods  are  entirely  different 
from  those  of  higher  groups  of  animals.  Vision  is  compound, 
the  eyes  being  made  up  of  thousands  of  minute  units,  termed 
ommatidia,  each  ommatidium  having  a  complex  structure  (Fig. 
75).  The  facets  (cornea),  like  mosaics,  form  the  outer  cuticle  of 
the  eye. 

REPRODUCTIVE  SYSTEM. — The  sexes  are  separate  in  the  ma- 
jority of  Crustacea,  and  the  gonads  are  relatively  much  larger 
than  in  the  earthworm.  The  testes  are  long,  beaded  organs, 
white  in  color,  lying  dorsally  to  the  hepato-pancreas,  one  on  each 
side  of  the  dorsal  blood  system.  The  male  gonoduct  or  vas 
deferens  originates  about  two-thirds  of  the  length  from  the 
anterior  end,  and  runs  downward  through  the  body  cavity  to 


182 


HOMOLOGY 


open  to  the  outside  on  the  basal  segment  of  the  fourteenth  ap- 
pendage (Fig.  68,  14).  From  this  opening  the  spermatozoa, 
packed'  together  in  bundles  called  spermatophores,  are  caught 
by  the  tubular  exopodites  of  the  fifteenth  pair  of  appendages, 
and  placed  on  the  genital  groove  of  the  female  during  copulation. 
The  ovaries  are  similar  in  general  shape,  and  in  position,  but 
are  bright  orange  in  color,  and  the  female  gonoduct  or  oviduct, 
while  it  originates  in  the  same  relative  position,  opens  to  the 
outside  on  the  basal  segment  of  the  twelfth  appendage.  The 
eggs  are  fertilized  as  they  pass  out,  and  are  covered  with  a 
gelatinous  mucus  by  which  they  stick  to  the  hairs  bordering  the 
swimmerets  or  abdominal  appendages.  Thousands  of  them 

become  thus  attached,  to 
be  swayed  back  and  forth 
by  the  movements  of  the 
swimmerets  during  the 
early  stages  of  develop- 
ment. 

DEVELOPMENT  AND 
METAMORPH  osis. — The 
development  of  the  earth- 
worm, as  of  Hydra,  begins 

FIG.  77. — A  typical  nauphus  larva  of  the  cop-      .  ... 

epods  with  three  pairs  of  appendages.        With    the    division    of    the 

egg  cell  into  two  cells  or 

blastomeres,  each  with  one-half  of  the  fertilization  nucleus. 
Development  of  the  lobster  begins  with  the  division  of  the 
nucleus,  without  division  of  the  egg  substance.  The  second, 
third,  etc.,  up  to  the  eighth  division  are  the  same,  a  multinu- 
cleated  cell  resulting,  in  which  the  nuclei  arrange  themselves 
around  the  periphery  of  the  egg.  Then  the  outer  zone  of  proto- 
plasm divides  around  each  nucleus,  the  cleavage  planes  passing 
radially  toward  the  egg  center  (Fig.  76).  This  type  of  cleavage 
is  characteristic  of  the  arthropods,  and  is  called  meroblastic,  as 
opposed  to  holoblastic.  The  yolk  is  collected  in  the  center  of 
the  egg,  which  for  this  reason  is  called  centrolecithal. 

Metamorphosis. — In  the  more  generalized  types  of  Crustacea 
this  method  of  cleavage  leads  to  the  formation  of  a  free  living 


DEVELOPMENT  OF  THE  LOBSTER 


183 


embryo  or  larva,  termed  the  nauplius.  This  larva  has  little 
resemblance  to  the  parent,  consisting  of  a  small  ovoidal  body, 
with  mouth,  three  pairs  of  biramous  appendages  and  a  median 
unpaired  simple  eye  (Fig.  77).  The  appendages  are  the  first 
three  pairs  of  the  adult,  and  in  this  stage  have  little  similarity  to 
the  later  antennules  antennae  and  mandibles.  Each  consists 
of  exopodite,  endopodite  and  protopodite,  which,  with  develop- 
ment of  the  larva,  become  transformed  into  the  specialized 
organs  of  the  head. 


FIG.  78. — "Mysis"  stage  in  the  development  of  the  lobster;  a  stage  in  which 
the  thoracic  appendages  are  all  biramous  (cf.  Fig.  87).     (From  Herrick.) 

Growth  of  the  larva  results  in  elongation  of  the  body  and  the 
formation  of  somites  at  the  posterior  end.  The  terminal  somite 
is  formed  first,  and  new  somites  are  added  by  a  process  of  growth, 
analogous  to  budding,  which  occurs  between  this  terminal  so- 
mite and  the  body.  After  each  somite  is  .thus  formed,  paired 
biramous  appendages  develop  on  it  as  outgrowths.  A  larval 
form  thus  develops  from  the  nauplius,  in  which  all  of  the  appen- 


184 


HOMOLOGY 


dages  are  provided  with  exopodites  and  endopodites  ("Mysis" 
stage,  Fig.  78). 

Even  in  the  early  stages  the  body  is  covered  by  a  carapace 
of  chitin.  This  is  by  no  means  as  heavy  and  tough  as  in  the 
adult;  nevertheless,  it  is  highly  resistant  and  unyielding. 
Growth  of  the  body  thus  results  in  an  organism  with  a  covering 
too  small  for  it — it  outgrows  its  clothes.  The  chitin  carapace 
then  splits  along  the  mid-dorsal  line,  and  the  organism  detaches 
its  tissues  from  the  exo-skeleton  and  pulls  itself  out  of  its  cramped 
quarters.  A  new  chitin  covering  is  then  secreted,  which  lasts 


FIG.   79. — Stages  in  the  early  development  of  lobster  homologous  with  the 
natiplius  larva  of  the  copepods.     (From  Parker  and  Haswell  after  Lang.) 


until  continued  growth  demands  a  new  change.  This  process  of 
moulting — termed  ecdysis — is  characteristic  of  the  Crustacea,  and 
continues  at  lengthening  intervals  throughout  the  life  of  the 
individual.  The  "soft-shell"  crab  has  just  shed  its  old  coat 
and  has  not  yet  produced  a  new  one. 

The  lobster's  development  differs  from  that  of  more  general- 
ized Crustacea,  in  that  the  embryo  does  not  leave  the  egg  mem- 
brane as  a  nauplius  larva,  but  continues  its  embryonic  devel- 
opment within  the  egg  membrane,  until  it  has  grown  into  the 
general  form  of  the  parent.  It  then  leaves  the  egg  case  (Figs. 
79,  80),  and  grows  by  successive  moults  into  the  adult. 


HOMOLOGY  AND  ADAPTATION 


185 


II.  GENERAL    BIOLOGICAL    INTEREST    OF    THE    LOBSTER 

The  structures  and  life  history  of  the  lobster  teach,  by  anal- 
ogy, the  story  of  evolution.  Structural  adaptations  of  animals 
to  different  modes  of  life,  interpreted  on  the  principle  of  homol- 
ogy,  furnish  evidence  of  the  origin  of  species  from  generalized 
types.  The  appendages  of  the  lobster  are  originally  all  alike, 
and  of  a  primitive  biramous  type.  From  this  primitive  type  by 


FIG.  80. — A  young  lobster  leaving  the  egg  case  (on  left).     (From  Herrick.) 

various  modifications,  several  different  forms  of  the  adult  ap- 
pendages are  derived.  These  appendages,  furthermore,  are 
utilized  for  different  purposes,  thus  illustrating  the  principle  of 
adaptation — a  generalized  type  of  organ  may  become  adapted  to 
several  different  kinds  of  uses. 

What  happens  among  homologous  parts  in  the  individual 
lobster  can,  theoretically,  take  place  in  allied  organisms  of  a 
given  group,  although  the  process  cannot  be  watched  as  it  can 
be  in  the  lobster.  We  find,  in  existing  animals,  structural 
adaptations  which  we  can  interpret  best  on  the  theory  of  com- 
mon origin.  Thus,  in  this  one  group  of  Crustacea  to  which  the 


186  HOMOLOGY 

lobster  belongs,  we  find  that  eight  pairs  of  thoracic  appendages 
is  the  rule.  In  the  lobster,  we  see  that  five  of  these  pairs  are 
adapted  for  walking  or  locomotion,  and  three  of  them  for  assist- 
ing in  procuring  and  manipulating  food.  So  too,  the  crab,  or 
shrimp,  and  many  allied  forms  have  the  same  distribution  of  the 
thoracic  appendages,  and  zoologists  group  them  together  as  an 
order  of  Crustacea,  called  Decapoda.  In  other  groups,  however, 
we  find  different  distributions  of  the  eight  pairs  of  thoracic  ap- 
pendages. One  such  group  has  only  three  pairs  of  walking  legs; 
the  remaining  five  are  adapted  for  food  manipulation  (Order 
Stomatopoda) .  In  another  group,  all  eight  are  rudimentary, 
none  being  developed  for  walking  (Order  Cumacea),  while  in 
another,  seven  of  the  eight  pairs  are  developed  for  walking,  and 
only  one  pair  serves  for  food  manipulation  (Order  Arthrostraca) . 
The  assumption  is  made  that  all  of  these  different  types  of 
Crustacea,  because  of  their  striking  similarities,  must  be  closely 
related,  and  must  have  had  a  descent  from  common  ancestors. 
Such  ancestors  could  not  have  been  more  specialized  than  are 
these  types  today;  they  must  have  been  more  generalized  forms, 
from  which  different  lines  of  adaptation  could  come. 

Such  generalized  ancestral  forms  of  the  Crustacea  are  repre- 
sented among  existing  types,  which  form  a  sub-class  (Entomos- 
traca)  of  the  Crustacea.  Their  appendages  and  somites  are 
more  numerous  than  twenty  pairs,  and  the  appendages  are  of  the 
primitive  biramous  type.  How  the  more  specialized  forms  of 
Crustacea  were  derived  from  these  more  generalized  types  is  a 
matter  of  speculation,  involving  the  factors  of  inheritance  and 
evolution  which  will  be  considered  in  a  subsequent  chapter. 

III.  INSECTS 

Another  series  of  illustrations  of  homology  may  be  found  in 
the  group  of  insects,  of  which  more  than  200,000  are  known.  In 
these  myriads  of  forms,  the  adaptations  of  wings  and  mouth 
parts  are  particularly  striking. 

Superficially,  the  insects  are  so  similar  to  Crustacea  that  form- 
erly they  were  all  classed  together  in  the  common  phylum 


INSECTS 


187 


Arthropoda.  Some  of  the  insects,  however,  have  quite  as  close 
an  affinity  to  the  annelid  worms,  one  genus,  Peripatus,  having 
many  annelid  characteristics.  Biologists,  therefore,  agree  in 
making  Crustacea  and  insects  independent  phyla,  with  common 
ancestors  in  annelid-like  forms. 

Like  Crustacea,  the  insect  body  is  composed  of  somites  which 
are  regionally  fused  to  form  more  or  less  independent  head, 


FIG.  81. — A  cockroach,  from  the  ventral  surface. 

thorax  and  abdomen.  The  head  consists  of  five  somites,  the 
thorax  of  three,  and  the  abdomen  of  eleven  or  less,  the  number  of 
somites  being  highly  variable  in  the  abdomen,  but  fixed  in  head 
and  thorax.  The  head  always  bears  compound  eyes  and,  very 
often,  simple  eyes  in  addition.  It  also  carries  one  pair  of  anten- 
nae, and  two  pairs  of  pre-maxillae  (Fig.  81).  In  the  cockroach, 
the  latter  are  united  to  form  a  labrum  overhanging  the  mouth 
(Fig.  82).  The  head  also  bears  one  pair  of  mandibles  or  jaws, 
and  two  pairs  of  maxillae.  In  different  orders  of  insects  these 
mouth  parts  are  adapted  for  different  modes  of  nutrition.  For 


188 


HOMOLOGY 


biting  and  chewing  in  orthoptera  (grasshoppers,  cockroach,  etc.), 
coleoptera  (beetles),  hemiptera  (bugs),  and  hymenoptera  (ants, 
bees  and  wasps) ;  for  sucking  or  licking,  diptera  (flies,  mosqui- 
toes, etc.),  lepidoptera  (butterflies,  moths),  and  neuroptera 
(dragon  flies,  etc.) .  Just  as  we  may  trace  homologies  of  the  crus- 
tacean appendages,  so  we  may  trace  the  homologous  parts  of 
different  insects  in  which  the  appendages  are  adapted  for 
different  functions  (Fig.  82). 


FIG.  82. — Homologous  mouth  parts  of  cockroach  (left),  bee  (center)  and  mos- 
quito (right).     (Combination  of  figures  from  Hertwig.) 

The  thorax  of  the  cockroach  consists  of  three  fused  somites, 
termed  the  pre-,  meso-,  and  meta-thorax,  and,  as  in  the  lob- 
ster, it  bears  the  most  important  organs.  Each  somite  carries 
one  pair  of  legs,  and  these  three  pairs  of  legs  are  so  constant  in 
the  insects  that  the  phylum  is  sometimes  called  the  Hexapoda. 
These  legs  are  adapted  for  many  different  activities. 

The  thorax  also  carries  the  wings.  These  are  thin  bags  of 
cuticle  drawn  out  from  the  dorsal  angles  of  the  meso-  and  meta- 
thorax,  which  become  expanded  and  stiffened  in  the  air,  and 
are  the  most  characteristic  of  the  external  appendages  of  insects, 
distinguishing  them  from  all  other  animal  forms.  The  wings, 
like  other  appendages,  are  also  subject  to  wide  variations  in 
form  and  function. 


ADAPTATIONS  IN  INSECTS  189 

The  internal  organs  of  insects  are  especially  adapted  for  an 
aerial  mode  of  life.  The  most  characteristic  are  adaptations 
for  breathing  air.  There  are,  obviously,  two  possible  different 
ways  in  which  organs,  tissues,  and  cells  of  the  body  may  obtain 
fresh  oxygen ;  each  tissue  may  get  it  directly  from  the  outside 
by  osmosis,  as  in  the  coelenterates  and  earthworms,  or  each  cell 
may  get  it  from  some  specially  modified  oxygen-carrying  agent. 
The  blood  vascular  system  forms  the  agent  in  the  majority 
of  higher  types,  but  in  the  insects  the  blood  system  has  no  such 
functions,  and  in  many  cases  is  absent  altogether.  Air  is  car- 
ried from  the  atmosphere  directly  to  the  tissues  and  cells  by 
special  tubes  called  tracheae,  which  form  a  complicated  system 
or  branching  system  of  vessels  distributed  throughout  the  body. 
The  main  trunks  end  in  external  openings  termed  spiracles, 
which  may  be  variously  placed  in  different  types  of  insects.  In 
some  cases,  there  is  only  one  pair  of  such  openings;  again  there 
may  be  a  pair  to  each  somite  of  the  abdomen  and  thorax 
(cockroach,  grasshopper),  or  many  openings  may  be  distributed 
about  the  body. 


CHAPTER  VIII 
PARASITISM:  PHYSIOLOGICAL  ADAPTATION 

A.  THE  TAPEWORM,  TAENIA  SP. 

Many  types  of  adaptation  can  be  traced  back  directly  to  the 
effects  of  the  environment.  These  may  be  either  structural  or 
functional  or  both.  A  tapeworm  has  no  mouth  or  digestive 
tract,  but  obtains  its  food  by  absorption  of  dissolved  proteins 
from  the  host.  It  has  little  need  for  movement — if  it  were 
necessary  for  it  to  move,  it  could  not  do  so  easily,  for  the  body 
musculature  is  inadequately  developed.  It  might  be  inferred 
from  its  position  in  the  digestive  tract  that  such  a  parasite  would 
need  some  apparatus  of  attachment.  Suckers  and  hooks  are 
developed  for  this  purpose.  Absence  of  muscular  develop- 
ment indicates  lack  of  need  for  nervous  system.  The  nervous 
system  is  most  primitive.  So,  too,  are  organs  of  excretion.  All 
of  these  structural  features  indicate  an  adaptation  for  the  par- 
ticular mode  of  life  of  an  intestinal  parasite.  Physiological 
adaptations  must  also  have  been  developed  with  the  change 
from  independent  individualism  to  dependent  association. 
The  loss  of  digestive  tract  could  not  have  occurred  in  the  an- 
cestry of  our  cestode,  so  long  as  there  was  need  of  it  for  life  of 
the  worm  (Fig.  83). 

The  greatest  physiological  adaptation,  however,  is  apparent  in 
the  reproductive  system.  The  entire  construction  of  the  tape- 
worm seems  bound  up  with  this  particular  activity.  The 
young  worm,  attached  to  the  intestinal  wall,  grows  by  ab- 
sorption of  food  digested  and  prepared  for  assimilation  by  the 
functioning  digestive  cells  of  the  host.  The  first  trace  of  repro- 
duction is  the  formation  of  a  somite-like  bud  at  the  posterior  end 
of  the  parasite.  Continued  growth  involves  continued  new  bud- 
formation  with  enlargement  of  the  older  buds,  until  a  long  chain 

190 


THE  TAPEWORM 


191 


of  similar  buds  growing  from  the  attached  "head"  end  (called 
scolex)  results.  The  completed  worm  then  has  a  superficial 
resemblance  to  a  segmented  form  such  as  an  annelid,  but  the 
resemblance  is  only  superficial,  for  the  segments  (called  pro- 
glottids)  are  not  typical  metameres  or  somites  and  have  little 


FIG.  83. — Taenia  sollum.    A,  Entire  tapeworm  with  proglottids;  £,  scolex  with 
sucking   discs   and   crown   of  hooks.     (From  Leuckart.) 

organic  relation  to  the  whole  worm.  Each  segment  has  a  com- 
plete set  of  reproductive  organs  which  are  quite  as  complex  as 
the  reproductive  organs  of  annelids  or  other  animals  of  similar 
grade  (see  Fig.  84).  When  mature,  the  proglottids  are  de- 


192 


PARASITISM 


tached  from  the  end  of  the  tapeworm  and  are  defecated  with  the 
faeces  of  the  host  to  the  outside.  Each  proglottid  has  the  power 
to  produce  thousands  of  eggs  which  are  fertilized  when  mature, 


FIG.  84. — A  single  proglottid  of  Taenia  solium  enlarged  to  show  the  reproductive 
organs.     (From  Leuckart.) 

and  stored  up  in  the  uterus  of  the  proglottid,  ready  for  develop- 
ment. When  detached,  a  ripe  proglottid  then  has  thousands  of 
embryos,  each  capable  of  giving  rise  to  a  new  tapeworm.  But 
these  are  deposited  with  the  faeces,  and  before 
they  can  develop  into  a  new  Taenia  must 
undergo  partial  development  in  the  pig.  In 
one  way  or  other,  they  find  their  way  into 
the  food  of  a  pig;  the  embryos  are  liberated 
by  action  of  the  pig's  digestive  fluids,  and 
when  liberated  make  their  way  through  the 
walls  of  the  digestive  tract  into  the  muscles 
of  the  pig.  Here  their  development  is  ar- 
rested, and,  as  cysticerdds  or  bladder-worms, 
they  give  rise  to  what  is  called  measly  pork. 
T-,  "  3~1".T.  Such  pork  eaten  in  an  uncooked  state  is  a 

rlG.     55. — iTlCnind 

siralis^  encysted  in  source  of  human   infection.      The  bladder- 
Hertvdg^after  BoasT  worms  are  freed  in  the  digestive  tract,  be- 
come  attached    as   scolecids    to   the    lining 
epithelium,  and  begin  to  bud  out  proglottids. 

Here,  there  is  a  very  characteristic  physiological  adaptation, 
in  which  the  difficulties  of  maintaining  the  species  are  balanced 
by  the  enormous  number  of  embryos  formed. 


SYMBIOSIS,  COMMENSALISM,  PARASITISM        193 

In  a  similar  way,  thousands  of  species  of  animals  become 
adapted  to  a  parasitic  life  in  different  types  of  host.  Further- 
more, there  are  different  grades  of  parasitism;  some  are  obliga- 
tory parasites,  requiring  a  particular  host  and  a  particular  organ, 
very  often,  of  that  host.  Others  are  facultative  parasites,  not 
absolutely  dependent  on  a  given  host,  but  capable  of  living  in 
such  a  host  if  chance  brings  them  there.  Thus  the  round  worm, 
Trichina,  is  an  obligatory  parasite  of  man,  and  a  facultative 
parasite  of  domesticated  animals,  including  the  pig.  The 
embryos  are  eaten  with  infected  meat;  liberated  in  the  human 
intestine,  they  penetrate  the  walls  of  the  digestive  tract  and 
multiply  in  the  body  cavity,  ultimately  penetrating  muscle 
bundles  where,  in  the  muscle  cells,  they  finally  encyst.  If  the 
unfortunate  victim  does  not  die  from  trichinosis  before  such 
encystment  occurs,  recovery  is  possible,  for  once  encysted, 
the  parasites  do  no  further  damage.  Trichinosis,  however,  is 
usually  fatal,  the  infected  muscles  of  the  victim  often  containing 
millions  of  the  parasites  (Fig.  85) . 

B.  ANIMAL  ASSOCIATIONS 

Animals  of  different  kinds  may  live  together  in  harmony  and 
without  ill  effects  on  either;  or  different  types  of  living  organ- 
isms may  live  together  for  mutual  benefit.  Such  a  form  of 
partnership  is  called  symbiosis,  an  example  of  which  we  have 
seen  in  the  case  of  Hydra  viridis — or  this  association  may  become 
obligatory,  so  that  neither  organism  can  live  without  the  other. 
Where  the  association  does  not  confer  mutual  benefit,  or  any 
obvious  advantage  to  both,  we  speak  of  the  association  as  com- 
mensalism.  A  good  example  of  this  is  the  union  of  the  glass 
sponge,  Euplectella,  and  a  crustacean;  a  pair,  male  and  fe- 
male, enter  the  pores  of  the  sponge  in  the  larval  stage,  and 
grow  to  adult  size  within  the  chosen  prison,  which  they  cannot 
leave  after  they  grow  up  (Fig.  86).  Numerous  examples 
may  be  found,  in  the  human  intestine,  of  both  commensalism 
and  symbiosis;  many  innocuous  protozoa  and  bacteria  live  there, 
while  many  bacteria  also  are  symbionts  which  play  an  important 


194 


PARASITISM 


FIG.  86. — A  glass^  sponge  (Euplectella)  with  commensal  Crustacea,  male  and 
female,  in  the  sponge  cavity;  the  female  is  carrying  eggs. 


IMMUNITY  195 

part  in  intestinal  digestion,  thereby  contributing  to  the  func- 
tioning activities  of  the  host  while  thriving  on  the  products  of 
digestion  in  the  intestine.  Parasites,  finally,  live  at  the  expense 
of  their  hosts.  These  are  all  illustrations  of  physiological  adap- 
tations on  the  part  of  symbionts,  commensals,  or  parasites, 
but  adaptations  do  not  stop  here. 

C.  ADAPTATIONS  AGAINST  PARASITES 

Parasites,  especially  some  forms  of  bacteria,  give  off  prod- 
ucts in  the  form  of  nucleo-proteins  or  other  chemical  com- 
pounds, which  act  as  poisons  on  the  host's  cells  and  tissues. 
Sometimes,  as  in  typhoid  fever  or  cholera,  these  poisons  from 
the  intestine  are  absorbed  into  the  vascular  system  and 
are  carried  to  all  parts  of  the  organism.  The  action  of  such 
poisons  differs  in  different  cases.  Very  frequently  the  proto- 
plasm and  walls  of  the  cells  of  the  digestive  tract  are  dis- 
solved, a  phenomenon  known  as  lysis  (hence  cytolysis,  hemo- 
lysis,  karyolysis,  etc.)  and  local  or  distributed  areas  of  func- 
tioning organs  are  ulcerated  and  destroyed,  leading  to  various 
forms  of  enteritis  or  intestinal  inflammation.  Or  the  parasites 
may  become  localized  in  other  parts  of  the  body,  pneumococcus 
in  the  lungs  giving  acute  congestion  characteristic  of  pneu- 
monia; or  the  bacillus  of  tuberculosis  in  the  lungs,  which  forms  a 
poison  causing  destruction  of  the  lung  cells  as  in  consumption. 
The  organisms  of  tonsilitis  and  diphtheria  accumulate  in  the 
throat  and  give  off  poisons  which  act  on  the  entire  system; 
the  organisms  of  smallpox  and  scarlet  fever  collect  in  the  skin, 
those  of  hydrophobia  in  the  nerve  cells  of  the  peripheral  and 
central  nervous  system,  while  some  find  their  way  into  the  blood 
and  multiply  there;  for  example,  Streptococcus  and  Staphylo- 
coccus  cause  blood  poisoning  or  malarial  organisms  cause 
malaria. 

These  various  parasites  have  become  adapted  to  this  parasitic 
mode  of  life  in  the  human  organism.  They  live  at  the  expense 
of  the  latter,  and  in  the  course  of  their  various  metabolic  ac- 
tivities they  give  off  substances  which  interfere  with  the  nor- 
mal activities  of  metabolism  of  the  host,  usually  by  the  direct 


196  PARASITISM 

or  indirect  destruction  of  organ  cells  by  which  normal  functions 
are  carried  on.  The  result  is  disorder  in  the  physiological 
balance  of  the  human  organism,  leading  to  morbid  symptoms, 
and,  if  uncontrolled,  to  death. 

Moreover,  just  as  these  parasites  have  become  adapted  to  a 
new  mode  of  life  in  the  host  organism,  so  the  host  organism  has 
become  physiologically  adapted  to  resist  them.  These  adapta- 
tions are  (i)  physical,  through  the  activity  of  white  blood  cells  or 
leucocytes  (phagocytes),  and  (2)  chemical,  through  the  forma- 
tion of  chemical  bodies  which  counteract  the  poisons  created  by 
the  parasites. 

(i)  Phagocytosis. — If  we  inject  a  bit  of  capsicum  into  the  skin 
of  a  salamander  or  other  amphibian,  the  result  is  a  collection  of 
blood,  or  inflammation,  in  the  vicinity.  If  the  experiment  is 
made  on  the  web  of  the  foot,  and  the  foot  fixed  under  the  micro- 
scope, the  course  of  the  blood  in  the  veins  and  in  the  capillaries 
can  be  easily  watched.  From  time  to  time  white  or  colorless 
cells  come  along,  hesitate  in  the  blood  flow,  stop  and  then 
begin  to  work  through  the  walls  of  the  capillary.  They  pass 
through  this  wall  and  into  the  surrounding  fluids,  the  process 
of  migration  being  known  as  diapedesis.  Thus  by  amoeboid 
motion  they  move  toward  the  seat  of  irritation,  and  if  yeast 
cells  or  powdered  carmine  be  injected  in  the  skin,  the  white  cells 
can  be  observed  to  engulf  them  exactly  as  an  amoeba  takes  in 
food.  These  white  cells  are  the  phagocytes  of  the  blood — mi- 
crophages  and  macrophages — and  their  function  is  to  surround 
and  engulf  any  foreign  bodies  or  irritating  substances  in  the 
organism.  This  function  is  phagocytosis. 

In  a  similar  manner,  the  phagocytes  may  attack  and  engulf 
bacteria  or  other  harmful  foreign  objects.  Once  engulfed,  they 
are  digested  by  intracellular  digestion  in  the  same  way  that 
amoeba  engulfs  and  digests  living  food.  The  phagocytes,  there- 
fore, contain  some  digestive  substance  fatal  to  bacteria,  pro- 
vided the  bacteria  are  engulfed  by  them,  and  just  as  alcoholic 
fermentation  is  possible  through  the  action  of  zymase  without 
the  living  yeast  cell,  so  extracts  of  phagocytes  would  be  capable 
of  destroying  bacteria.  This  apparently  happens  under  con- 


ANTI-BODIES  AND  IMMUNITY  197 

ditions  of  disease.  The  invading  bacteria,  in  some  cases,  pro- 
duce poisons  which  destroy  cells  of  the  body  and  phagocytes; 
with  their  destruction  the  contained  digestive  fluids  or  chemicals 
are  liberated;  these  in  turn  react  on  the  bacteria  and  kill  them. 
Thus,  if  rabbits  are  inoculated  with  the  bacilli  of  anthrax,  the 
parasites  multiply  in  the  blood,  over-run  every  organ  of  the 
body  and  ultimately  kill  the  rabbit.  If,  however,  the  bacilli 
are  placed  in  rabbits'  blood  that  has  been  drawn  out  into  a  test- 
tube,  the  bacteria  are  killed  (Nuttall  and  Buchner).  This 
result,  as  it  is  usually  interpreted,  is  due  to  the  death  and  disin- 
tegration of  phagocytes,  whereby  some  chemical  substance 
(called  alexine  by  Buchner),  which  is  fatal  to  the  bacteria,  is 
liberated  from  the  disintegrating  protoplasm.  Alexine  is 
supposed  to  be  the  same  substance  which  brings  about  digestion 
of  bacteria  ingested  by  living  phagocytes. 

Some  types  of  bacteria  in  the  blood  are  attacked  and  killed 
by  the  phagocytes.  This  was  demonstrated  by  Metschnikoff 
who,  using  a  strain  of  bacteria  which  are  thus  killed  in  the  blood, 
enclosed  some  in  collodion  sacs  which  he  placed  in  the  body  cavi- 
ties of  different  animals.  These  sacs  allowed  the  free  inter- 
change of  fluids,  including  bacterial  products  and  various  sub- 
stances contained  in  the  blood,  but  the  bacteria  themselves 
could  not  get  through,  nor  could  the  phagocytes  reach  the 
bacteria  which  lived  and  thrived  in  the  collodion  sacs. 

Again,  some  types  of  bacteria  in  the  blood  are  not  touched 
by  phagocytes  under  ordinary  conditions,  but  if  animals  con- 
taining such  bacteria  are  inoculated  with  phagocyte-free  blood, 
which  is  immune  to  these  bacteria,  the  phagocytes  immediately 
devour  them.  Something  in  the  serum  has  produced  a  change 
in  the  bacteria  which,  while  it  leaves  the  bacteria  uninjured  so 
far  as  their  vital  processes  are  concerned,  renders  them  suscep- 
tible to  attack  by  phagocytes.  Wright,  who  discovered  this 
phenomenon,  gives  the  name  opsonin  to  the  substance  which 
makes  bacteria  susceptible  to  phagocytes  (opsono — I  prepare 
the  food). 

(2)  Anti-bodies  and  Immunity. — While  phagocytes  are  thus  a 
potent  physiological  adaptation  for  protecting  the  organism 


198  PARASITISM 

against  disease,  they  do  not  form  the  sole  means  of  protection. 
The  phenomena  of  immunity  furnish  another  and  more  subtle 
illustration  of  physiological  adaptation. 

Everyone  is  familiar  with  the  ordinary  facts  of  immunity 
from  disease.  In  a  community  in  which  some  contagious  or  in- 
fectious disease  is  epidemic,  some  individuals  do  not  acquire  the 
disease,  if  exposed  to  it.  These  individuals  are  said  to  be  im- 
mune, and  of  these  there  are  usually  two  classes;  in  one  class,  the 
individuals  have  never  had  the  disease,  but  enjoy  what  is  called 
natural  immunity.  Again,  other  individuals  take  the  disease  but 
have  it  in  mild  form;  they  are  said  to  be  slightly  susceptible  to 
the  disease,  but  have  sufficient  natural  immunity  to  make  it  a 
light  case.  Still  others  are  highly  susceptible,  and  succumb. 

In  a  similar  way,  entire  races  may  be  naturally  immune  to 
diseases  that  are  ordinarily  fatal  to  other  races.  Thus  the 
horse  and  ass  are  highly  susceptible  to  the  organism  of  glanders, 
but  cattle,  sheep  and  fowls  can  be  injected  with  large  doses  of 
the  glanders  organism  without  ill-effects — they  are  naturally 
immune.  Many  diseases  of  lower  animals,  such,  for  example, 
as  hog  cholera,  swine  plague,  chicken  cholera,  mouse  septi- 
caemia, etc.,  fatal  to  these  animals,  are  harmless  to  human 
beings.  On  the  other  hand,  some  diseases  of  man,  like  scarla- 
tina, whooping  cough,  yellow  fever,  etc.,  are  harmless  to  lower 
animals,  while  some  others  like  anthrax,  tuberculosis,  etc.,  are 
equally  dangerous  both  to  man  and  lower  animals. 

Most  of  us  have  passed  through  the  ordinary  diseases  of 
childhood  ourselves — whooping  cough,  chicken-pox,  mumps, 
and  some  through  smallpox  and  scarlet  fever,  none  of  which  we 
expect  to  have  a  second  time  because  of  the  acquired  immunity 
which  these  diseases  have  left  in  us. 

Again,  most  of  us  have  been  vaccinated  against  smallpox,  and 
some  of  us  against  typhoid  fever,  and  neither  of  these  diseases 
may  be  expected  after  such  vaccination,  which  has  given  us  an 
acquired  immunity.  This  vaccination  has  produced  changes  in 
the  physiological  mechanism  similar  to  the  changes  produced 
by  the  diseases  themselves.  Thus  acquired  immunity  may  be 
of  two  types  (a)  active  immunity,  through  experience  of  the  dis- 


THE  MECHANISM  OF  IMMUNITY  199 

ease  or  by  vaccination  with  organisms  which  produce  the  disease, 
or  by  their  products,  and  (b)  passive  immunity,  by  injection  of 
a  serum  from  an  actively  immunized  animal,  carrying  with  it 
certain  substances  by  which  protection  is  conferred.  If 
organisms  are  introduced  with  vaccination,  they  are  rendered 
comparatively  harmless  by  preliminary  treatment.  Thus, 
experience  has  shown  that  the  organism  of  smallpox  is  rendered 
harmless  to  man  by  passing  it  through  the  calf,  which  is  only 
mildly  suceptible  to  the  disease.  When  recovered  from  the 
calf,  the  organisms  (virus) ,  are  weakened  in  such  a  way  that  upon 
inoculation  into  man  they  produce  only  a  local  disturbance,  but 
enough  to  change  the  chemical  make-up  of  the  blood,  which  will 
then  protect  the  body  against  smallpox  for  years. 

A  very  striking  case  of  passive  immunity  is  furnished  by  the 
modern  treatment  of  diphtheria.  The  ill  effects  of  the  disease 
are  due  to  poisons  produced  by  the  parasite  of  diphtheria— 
these  spread  through  the  victim,  and  by  their  cumulative  effect 
either  cause  death  or  stimulate  the  cells  of  the  body  to  produce 
an  antidote  in  sufficient  quantity  to  neutralize  the  poison. 
The  actual  existence  of  such  an  antidote  was  discovered  in 
1890  by  Kitasato  and  von  Behring,  and  named  by  them  an 
anti-body.  It  was  found,  furthermore,  that  lower  animals 
could  be  employed  as  the  source  of  the  anti-body.  The  horse, 
for  example,  may  be  inoculated  with  the  organisms  of  diphtheria 
—after  some  days  the  blood  of  the  horse  contains  quantities  of 
the  anti-body,  so  that  the  serum,  if  injected  into  a  human  vic- 
tim of  diphtheria,  counteracts  the  poison  produced  by  the 
diphtheria  organisms  of  the  victim.  It  is  a  case  of  acquired 
active  immunity  in  the  horse,  and  acquired  passive  immunity 
in  the  human. 

D.  THE  MECHANISM  OF  IMMUNITY 

What  is  the  nature  of  this  change  in  the  blood,  whereby 
organisms  or  their  poisonous  products  are  counteracted?  The 
fundamental  principle  underlying  immunity  is  that  the  blood 
contains  something  which  it  did  not  contain  before.  Sub- 
stances which  produce  this  change  are  called  antigens,  and 


200  PARASITISM 

the  new  responsive  bodies  are  called  anti-bodies.  Now  the 
facts  of  immunity  are  established  and  there  is  an  increasing 
multitude  of  such  facts,  but  the  explanations  are  purely  theo- 
retical. There  are  two  main  hypotheses  at  the  present  time ;  one 
is  MetschnikofFs  development  of  phagocytosis,  the  other  is  Ehr- 
lich's  famous  side-chain  hypothesis.  According  to  the  former, 
all  responses  of  anti-body  formation  to  antigens  take  place  in 
the  phagocytes,  of  which  there  are  two  kinds — microphages 
and  macrophages.  The  former  are  the  leucocytes  of  the  blood, 
which  engulf  bacteria  and  other  minute  bodies,  and  digest  them 
by  the  aid  of  an  enzyme  clled  microcytase.  The  latter  are  modi- 
fied organ  cells  of  the  body,  which  have  become  dissociated 
from  their  tissues,  and  roam  about  as  scavengers  in  the  blood 
supply,  producing  an  enzyme  called  macrocytase,  and  digesting 
larger  bodies  than  bacteria.  When  broken  down  under  the 
action  of  antigens,  they  liberate  chemical  substances  which 
form  the  counteracting  chemical  anti-body.  The  reaction 
thus  is  purely  chemical  or  physiologico-chemical  in  nature. 

In  Ehrlich's  theory  there  is  an  attempt  to  visualize  the  actual 
process  of  the  physiologico-chemical  action.  The  unknown 
myriads  of  molecules  which  make  up  protoplasm  are  imagined 
to  have  unsatisfied  groups  of  atoms,  ready  to  unite  with  food 
substances  or  other  substances  from  the  blood.  These  free 
groups  are  called  side-chains.  Instead  of  uniting  with  food 
substances,  one  or  many  may  unite  with  molecules  of  poison, 
which  are  thus  introduced  into  the  protoplasmic  substance, 
resulting  in  the  destruction  of  the  side-chains.  If  the  number 
of  such  molecules  of  poison  is  limited,  the  protoplasm  is  able 
to  regenerate  the  atom  groups  thus  used,  but  if  the  poison 
accumulates,  the  new  groups  are  combined  as  soon  as  formed, 
and  fatal  poisoning  results.  In  the  case  of  diphtheria,  the  horse 
undergoes  such  direct  poisoning,  but  not  extensive  enough  to 
produce  fatal  results.  Its  blood,  however,  becomes  loaded  with 
anti-bodies.  According  to  Ehrlich's  theory,  this  is  explained 
by  the  assumption  that  atom  groups  of  the  molecules  in  proto- 
plasm, when  thus  destroyed  by  the  poison  molecules,  are  regen- 
erated, and  these  regenerated  groups  are  cast  off  from  the  proto- 


EHRLICH'S  THEORY  OF  IMMUNITY  201 

plasm  into  the  blood  as  free  atom  groups,  which  are  then  capable 
of  uniting  in  the  blood  with  the  poison  molecules.  In  this 
way,  chemical  union  of  antigen  and  anti-body  takes  place  out- 
side or  apart  from  protoplasm,  and  when  thus  united,  the  poison 
is  made  harmless,  because  of  its  inability  now  to  unite  with  any 
protoplasmic  group — its  valencies  have  been  satisfied.  Fur- 
thermore, Weigert  has  shown  that  the  quantity  of  protective 
substances  in  the  blood  (anti-bodies)  is  out  of  all  proportion  to 
the  quantity  of  toxin  which  stimulated  the  reaction.  In  other 
words,  hyper-regeneration  follows  such  toxic  injuries  to  the 
protoplasm.  Thus,  in  the  case  of  diphtheria  it  has  been  shown 
that  one  unit  of  diphtheria  toxin  is  sufficient  to  produce  100,000 
units  of  anti-body.  Immunity,  therefore,  is  explained  by  Ehr- 
lich  as  the  condition  whereby  the  blood  is  loaded  up  with  free 
chemical  substances,  which  unite  with  and  render  harmless 
the  specific  poison  of  a  disease-causing  parasite.  It  is  a  most 
pregnant  theory,  and  has  been  developed  with  surprising 
ingenuity  to  satisfactorily  account  for  all  of  the  complications 
connected  with  zymotic  diseases.  One  only  of  these  compli- 
cations will  be  given  here,  as  the  subject  of  immunity  is  vast 
and  perplexing.  The  case  of  opsonin  formation  and  action  is  a 
relatively  simple  adaptation  of  the  theory.  Some  bacteria 
in  the  blood,  e.g.}  tubercle  bacilli,  are  relatively  unharmed  by  the 
phagocytes  under  normal  conditions,  but  if  immune  serum  be 
added,  the  bacteria  are  immediately  devoured,  or  in  some  cases 
dissolved  without  being  engulfed.  According  to  Ehrlich's 
theory,  there  is  no  chemical  attraction  or  proper  grouping  of 
atoms  in  the  bacterial  cell  to  enable  the  protecting  substances  to 
unite  with  them.  With  the  addition  of  immune  serum,  however, 
the  union  is  effected — the  substances  contained  in  it  being  able 
to  unite  with  both  the  bacteria  and  the  phagocyte.  In  such  a 
case,  the  phagocytes  or  dissolving  anti-bodies  form  the  com- 
plementj  the  molecules  of  immune  serum  form  the  connecting 
links,  and  are  known  as  amboceptors.  Hence,  without  the  am- 
boceptors,  the  anti-bodies  in  the  blood  are  unable  to  unite 
with  the  toxins,  any  more  than  pepsin  can  digest  proteins  in  an 
acid-free  medium. 


202  PARASITISM 

It  is  not  our  purpose  here  to  examine  deeply  into  the  secrets  of 
immunity — the  magnitude  of  the  subject  forbids  anything  more 
than  the  briefest  exposition  of  the  phenomenon  of  physiological 
adaptation,  which  immunity  suggests.  What  this  phenomenon 
means  to  the  organism  can  be  imagined,  when  the  same  indi- 
vidual becomes  successively  immunized  to  chicken-pox,  whoop- 
ing cough,  measles,  mumps,  smallpox,  and  half  a  dozen  other 
diseases.  The  fact  of  minute  reactions  bringing  about  great 
adaptations  against  disease  is  only  one  more  instance  of  the 
marvelous  powers  of  adaptation  which  protoplasm  manifests. 


CHAPTER  IX 
THE   PERPETUATION  OF  ADAPTATIONS 

WE  have  seen  that  the  appendages  of  the  Crustacea  are  built 
primarily  on  the  same  type  of  structure,  and  that,  in  different 
orders,  the  several  appendages  have  become  modified  in  one 
way  or  another  for  the  performance  of  different  functions.  In 
the  Malacostraca,  the  appendages  are  reduced  to  twenty  pairs 
in  all,  but  in  the  more  primitive  forms  included  in  the  group 
Entomostraca,  there  are  large  numbers  of  pairs  made  up  of 
primitive  biramous  appendages,  all  performing  practically  the 
same  functions.  The  twenty  pairs  (in  one  order,  Phyllocarida, 
there  are  twenty-one  pairs)  are  distributed  in  the  same  way  in 
all  of  the  Malacostraca  (Fig.  68) ;  five  belong  to  the  head,  eight 
to  the  thorax,  and  six  to  the  abdomen.  The  abdominal  ap- 
pendages retain  the  original  biramous  condition,  but  the  other 
thirteen  pairs  are  variously  modified  for  the  performance  of 
different  functions.  Examining  only  the  thoracic  appendages, 
we  find  one  order  (Schizopoda) ,  in  which  all  eight  pairs  are 
biramous  and  of  the  generalized  type,  all  serving  for  swim- 
ming (Fig.  87).  In  the  other  orders  they  are  adapted  for"  feeding 
and  walking  in  a  variety  of  ways.  In  the  order  Stomatopoda, 
five  pairs  are  modified  for  food  getting,  and  only  three  pairs  for 
walking;  in  the  order  Decapoda,  three  pairs  are  modified  for 
food  getting,  and  five  are  devoted  to  walking;  in  the  order  Cum- 
acea,  only  two  pairs  are  for  food  getting,  while  six  are  devoted 
to  walking;  and  in  the  order  Arthrostraca,  only  one  pair  is  for 
food  getting,  the  other  seven  for  walking.  In  all  of  these  cases, 
the  morphology  of  these  organs  indicates  that  the  most  highly 
differentiated  appendages  and  the  most  generalized  ones  are 
built  on  the  same  plan  of  structure.  The  study  of  the  develop- 
ment of  the  individual  crustacean  indicates  that  the  most 
highly  differentiated  appendages  appear  first  as  generalized 

203 


204         THE  PERPETUATION  OF  ADAPTATIONS 

biramous  structures  which,  with  growth,  lose  their  generalized 
structure  and  become  specialized.  In  different  regions  of  the 
same  organism  and  in  different  organisms,  therefore,  the  same 
generalized  type  may  become  adapted  for  quite  diverse  activities. 
Thus  the  fourth  pair  of  thoracic  appendages  in  Mysis  (Schizo- 
pod),  Squilla  (Stomatopod),  and  the  lobster  (Decapod)  are,  at 
some  period  in  development,  the  same  in  structure,  and  resemble 
one  another,  but  in  Mysis  they  remain  biramous  and  lamella- 
like;  in  Squilla  they  become  modified  into  characteristic  maxilli- 


FIG.  87.- — A  schizopod  (Gnathophausia  gigas)  with  permanent  biramous  tho- 
racic appendages  (cf.  schizopod  stage,  in  development  of  the  lobster,  Fig.  78). 
(From  Sars.) 

peds  or  food  getting  organs,  and  in  the  lobster  they  change  into 
the  powerful  offensive  and  defensive  chelate  walking  legs  char- 
acteristic of  the  Decapods.  Such  differences  form  the  basis  of 
animal  species. 

A.  ANIMAL  DESCENT 

The  lesson  taught  by  the  structures  and  functions  of  the 
lobster's  appendages  may  be  extended  to  all  animals.  As  the 
specialized  appendages  of  different  Crustacea  may  be  traced 
back  to  generalized  structures,  so  may  animals,  no  matter  how 
modified,  be  traced  back,  more  or  less  accurately,  to  more 
generalized  types  from  which  they  have  descended.  This 
descent  is  often  difficult  to  make  out,  for  some  of  the  primitive 
structures  may  have  become  so  modified,  through  functional 
activity  or  otherwise,  as  to  be  unrecognizable;  others,  like  the 


ADAPTATIONS  205 

exopodites  of  the  walking  legs  of  the  lobster,  may  have  dis- 
appeared entirely,  and  even  the  embryological  evidence  may 
have  been  eliminated  in  the  rapidity  of  development  of  the 
individual;  finally,  new  structures  may  have  appeared  without 
recognizable  homologies  in  the  primitive  forms.  All  of  these 
possibilities  offer  problems  for  the  student  of  animal  descent, 
and  their  solution  is  at  first  hypothetical,  such  hypotheses  being 
based  upon  different  biological  evidence,  sometimes  upon  the 
facts  of  comparative  anatomy,  sometimes  upon  transient  em- 
bryological structures,  sometimes  upon  geographical  distribu- 
tion, but  usually  upon  two  or  more  of  these  lines  of  evidence 
taken  together.  The  value  of  such  hypotheses  of  the  origin  of 
different  types  of  animals  depends  upon  the  amount  and  the 
nature  of  such  evidence,  which  may  be  so  convincing  as  to  make 
the  hypothesis  an  established  truth.  This  is,  indeed,  the  case 
with  the  theory  of  evolution  itself,  the  evidence  upon  all  sides 
being  so  conclusive  that  no  other  general  hypothesis  of  the 
origin  of  the  present  day  living  beings  is  tenable. 

It  is  within  the  memory  of  many  men  still  living,  that  the 
different  species  of  Crustacea  comprising  the  orders  mentioned 
above  would  have  been  interpreted  by  intelligent  people  as 
especially  created  types  of  animals  having  no  genetic  relation- 
ship. Today,  such  organisms  are  universally  believed,  by 
people  who  have  the  right  of  opinion,  to  be  blood  relations  and 
to  have  had  a  common  ancestry  from  generalized  Crustacea. 

B.  EVOLUTION 

The  change  in  point  of  view  was  due  to  the  acceptance  of  the 
doctrine  of  evolution  which,  in  its  modern  form,  had  its  start 
with  the  publication  of  Charles  Darwin's  book  on  The  Origin 
of  Species  in  1859.  This  volume  embodied  the  observations  of 
a  keen  naturalist,  running  over  a  period  of  thirty  years,  with  an 
argument  for  evolution,  through  natural  selection  in  the  struggle 
for  existence,  of  all  kinds  of  living  things. 

The  effect  of  this  one  book,  and  of  the  acrimonious  controversy 
which  it  brought  about,  was  a  revolution  in  human  thought. 


206          THE  PERPETUATION  OF  ADAPTATIONS 

Biologists  of  all  countries  and  thinking  men  of  all  walks  in  life 
were  drawn  into  the  controversy.  Living  nature  was  searched 
everywhere  for  evidence  bearing  on  evolution.  Habits  and 
instincts,  coloration  and  mimicry  of  animals,  living  and  fossil 
forms  were  studied  minutely  in  the  search  for  proof,  and  little 
by  little,  with  the  accumulation  of  facts  the  opponents  of  evolu- 
tion were  won  over,  until  finally  the  conception  of  evolution 
was  universally  accepted  as  the  explanation  of  the  origin  of 
modern  types  of  living  things. 

In  the  meantime,  however,  another  controversy  arose,  this 
time  among  biologists  themselves  who,  having  accepted  what  has 
taken  place  through  evolution,  did  not  agree  as  to  how  it  has 
taken  place  nor  in  regard  to  the  factors  involved.  Darwin 
believed  that  natural  selection,  by  which  those  organisms  best 
adapted  to  survive  in  the  struggle  for  existence  would  continue 
to  live  and  breed,  was  the  chief  means  of  the  maintenance  of 
diverse  types,  although  not  the  only  means.  Some  later 
biologists  believed  that  this  process  of  natural  selection  is 
itself  the  source  of  variations,  as  well  as  the  means  of  perpetu- 
ating them  after  adaptations  had  arisen,  useful  adaptations 
being  selected  and  conserved,  useless  adaptations,  being  a 
hindrance,  would  lead  to  extinction.  Still  other  biologists 
could  see  little  basis  for  the  origin  of  variations  on  Darwin's 
theory  of  natural  selection,  and  turned  back  to  the  view  advo- 
cated by  Lamarck  in  1815,  to  the  effect  that  animals  may  be- 
come changed  or  adapted  to  conditions  of  their  environment 
during  their  individual  lifetime,  and  then  transmit  such  ac- 
quired changes  or  adaptations  to  their  offspring.  These  Neo- 
Lamarckians  thus  believed  in  the  inheritance  of  acquired 
characteristics,  which  Darwin  himself  believed  might  play 
some  slight  role  in  the  origin  of  species. 

One  effect,  in  large  part,  of  this  controversy  among  biologists 
was  to  introduce  a  new  method  of  research  in  biological  science, 
and  experimental  biology  grew  up.  At  first,  animals  were 
mutilated  in  various  ways  to  see  if  such  mutilations  would  have 
any  effect  upon  the  offspring.  The  failure  of  such  experiments 
was  no  check  upon  the  use  of  the  experimental  method,  which 


EVOLUTION  207 

in  the  different  fields  of  experimental  zoology,  botany,  embryol- 
ogy, and  genetics  introduced  many  new  problems  for  solution, 
while  the  attempts  to  solve  them  threw  a  brilliant  flood  of  light, 
not  only  on  the  old  question  of  evolution,  but  over  the  entire 
field  of  biological  phenomena.  Indeed,  it  may  be  safely  stated 
that  all  of  the  great  strides  in  modern  biology  have  been  along 
the  lines  marked  out  through  use  of  experimental  methods. 

The  greatest  of  these  strides  has  been  taken  along  the  line  of 
heredity  or  genetics,  and  a  new  point  of  view  of  the  origin  of 
variations  has  resulted.  It  is  not  the  work  of  any  one  man, 
nor  has  the  advance  always  been  definite,  but  certain  names 
stand  out  prominently,  and  certain  achievements  mark  succes- 
sive outposts  of  advance. 

C.  CONFORMITY  TO  TYPE 

A  female  lobster  produces  thousands  of  eggs,  each  of  which, 
after  fertilization,  has  the  potential  of  a  new  adult  lobster,  and 
all  of  the  brood  are  essentially  similar  to  one  another  .  and 
similar  to  those  produced  by  other  lobsters.  If  one  or  more 
claws  of  two  parent  lobsters  are  cut  off  early  in  life  and  kept  cut 
off  after  successive  regenerations,  the  fertilized  eggs  resulting 
from  these  two  individuals  will  develop  again  into  normal  adult 
lobsters  with  perfect  appendages.  I  am  not  aware  that  such  an 
experiment  has  actually  been  made  with  lobsters,  but  it  has 
been  worked  out  in  so  many  other  cases  that  we  are  justified  in 
assuming  the  result  with  these  Crustacea.  The  main  point  is 
that  the  visible  changes  or  defects  of  the  parents  have  no 
apparent  effect  on  the  eggs  and  embryos  produced  by  them. 
In  other  words,  the  germ  cells  conserve  the  particular  type  of 
organism  producing  them,  not  necessarily  that  of  the  immediate 
parents,  but  of  the  race  to  which  the  parents  belong. 

If  the  various  types  of  Crustacea  which  are  known  today  have 
had  common  ancestral  forms  in  some  more  generalized  type, 
why  did  not  the  eggs  of  that  generalized  type  produce  organ- 
isms similar  to  the  parents,  and  when  and  how  did  changes 
occur?  It  is  the  old  problem  of  the  hen  and  the  egg;  the  egg 


208         THE  PERPETUATION  OF  ADAPTATIONS 

came  first,  but  the  hen  gradually  evolved  from  generalized  an- 
cestral forms  quite  dissimilar  from  the  modern  fowl.  How  were 
the  successive  changes  or  variations  impressed  on  the  egg,  until 
such  changes  became  normal  to  the  modern  types? 

D.  SOMATIC  AND  GERM  PLASM 

The  old  enigma  has  been  partly  solved  through  the  experi- 
mental method  in  modern  biology.  The  full  solution  is  indeed 
far  from  reached,  but  the  key  apparently  has  been  found.  As 
usual  in  science,  imagination  led  the  way  in  the  search  for  this 
key,  which  is  now  generally  believed  to  be  bound  up  in  the  con- 
ception of  the  germ  plasm  as  outlined  in  a  series  of  hypotheses 
by  Darwin,  Dalton,  Spencer,  Nageli,  and  especially  by  Weis- 
mann  in  his  famous  essays  on  Heredity.  According  to  Weis- 
mann's  conception,  our  lobster,  like  all  animals,  is  made  up  of 
two  kinds  of  protoplasm,  somatic  plasm  and  germ  plasm.  The 
somatic  plasm  forms  the  structures  of  the  individual,  including 
all  of  the  systems  of  nutrition  and  relation;  the  germ  plasm,  in 
the  female,  is  confined  to  the  eggs  and  to  the  endothelium  from 
which  eggs  are  formed,  and  in  the  male,  to  the  sperm  cells  and  to 
the  endothelium  from  which  sperm  cells  are  derived.  The  so- 
matic plasm  wears  out  and  ultimately  dies  from  old  age,  but  the 
germ  plasm  is  handed  down  and  continues  to  live  in  generation 
after  generation  of  descendants.  The  individual  thus  is  a 
nurse  or  carrier  of  the  potentially  immortal  germ  plasm;  his 
inheritance  comes  not  from  the  somatic  protoplasm  of  either 
parent,  but  from  the  germ  plasm  of  both,  and  here  is  the  secret 
of  the  conformity  to  type — the  race  is  preserved  in  the  germ 
plasm,  and  the  race  changes  with  changes  in  the  germ  plasm. 

The  development  of  this  idea  forms  one  of  the  most  interesting 
chapters  in  the  history  of  biological  science.  While  highly  specu- 
lative, especially  at  the  outset,  it  was  nevertheless  developed 
on  a  basis  of  facts.  These  facts  are  connected  with  the  phenom- 
ena of  mitosis  or  cell  division,  which  were  worked  out  by  many 
different  observers  during  the  two  decades  from  1870  to  1890. 
It  was  found  that  every  characteristic  type  of  animal  or  plant  cell 
reproduces  its  like  by  cell  division,  and  that  the  characteristic 


MITOSIS  209 

and  most  important  changes  of  the  cell  during  division  were 
connected  with  the  nucleus.  It  was  shown  that  the  chromatin 
of  the  nucleus  during  vegetative  stages  is  distributed,  in  the 
form  of  granules,  on  a  network  of  achromatic  material  called 
linin  (Fig.  88,  A)  ;  that  these  granules  collect  and  coalesce  in  one 
or  more  spirally  wound  threads  of  chromatin  called  the  spireme 
(Fig.  88,  B,  c);  that  these  spireme  threads  divide  longitudinally 
throughout  the  entire  length,  and  that  the  double  spireme  then 
segments  into  a  number  of  short  double  rods  called  chromo- 
somes (Fig.  88,  D,  E)  .  It  was  discovered  that  the  number  of  these 
chromosomes  is  always  the  same  in  individuals  of  the  same 
species  and  in  all  types  of  the  tissue  cells  of  the  same  individual. 
At  the  same  time,  it  was  shown  tiHat^the  chromosomes  collect 
in  the  center  of  a  peculiar  spindle-formed  body,  derived  from 
achromatic  material  of  the  cell,  and  having  characteristics  pecul- 
iar to  itself  in  the  form  of  centrosomes  at  the  poles  of  the  spindle, 
with  spindle  fibers  running  from  one  centrosome  to  the  other 
(Fig.  88,  D,  E,  F).  It  was  found  that  these  centrosomes  arise  by 
the  division  of  a  single  centrosome  lying  on  the  periphery  of  the 
nucleus,  and  by  separation  of  the  daughter-centrosomes  through 
an  arc  of  180°;  also  that  the  nuclear  membrane  disappears  at 
this  time,  while  new  fibers  (mantle  fibers)  grow  out  from  the 
centrosomes,  and  connect  with  the  chromosomes.  It  was  seen 
that  the  two  equal  parts  of  each  chromosome  then  separate  from 
one  another,  each  half  going  toward  one  of  the  two  centrosomes, 
so  that  the  entire  mass  of  chromatin  material  is  equally  divided 
between  the  two  daughter-nuclei  which  are  bounded  by  new  nu- 
clear membranes  (Fig.  89,  G,  H,  I,  j) .  It  was  noted,  finally,  that 
nuclear  division  is  completed  by  disintegration  of  the  daughter- 
chromosomes  into  the  distributed  chromatin  granules  character- 
istic of  the  vegetative  nucleus,  and  that,  after  this  nuclear 
division,  the  cell  body  divides  by  a  plane,  passing  through  the 
center  of  what  was  the  nuclear  division  figure. 

This  complicated  chain  of  processes  with  its  involved  activity 
of  chromatin,  centrosomes  and  spindle  fibers  was  named  karyo- 
kinesis  by  Schleicher  1878,  and  mitosis  by  Flemming  1882,  and 
both  names  are  found  in  current  literature. 


210 


THE  PERPETUATION  OF  ADAPTATIONS 


E 


FIG.  88. — Diagram  showing  the  early  stages  of  mitosis.  A,  Resting  cell  with 
reticular  nucleus  and  nucleolus;  at  c,  the  attraction  sphere  with  two  centrosomes; 
B,  early  prophase,  the  chromatin  in  the  form  of  a  continuous  thread  or  spireme; 
the  centrosomes  have  separated  forming  a  spindle  figure  between  them;  this  is 
the  amphiaster  (at  a) ;  C,  D,  two  later  stages  in  the  prophase  of  division  in  which 
the  spireme  is  divided  into  segments,  the  chromosomes;  E,  formation  of  the 
mitotic  figure  with  centrosomes  at  the  poles  and  with  the  chromosomes  split  longi- 
tudinally; F,  the  mitotic  figure  fully  established,  a,  Amphiaster;  ep,  equatorial 
plate.  (From  Wilson.) 


MATURATION  PHENOMENA  211 

Wilhelm  Roux,  speculating  on  the  significance  of  these 
nuclear  phenomena,  suggested  that  the  minute  and  exact  halv- 
ing of  these  fundamental  structures  of  the  cell,  and  the  com- 
plicated processes  by  which  this  halving  is  brought  about,  must 
have  some  important  bearing  on  deeper  biological  problems,  and 
he  concluded  that  karyokinesis  is  the  means  by  which  hereditary 
characteristics,  contained  potentially  in  the  chromosomes,  are 
distributed  to  all  cells  of  the  organism.  This  conception  in 
connection  with  the  germ  cells  was  taken  up  by  Weismann  and 
worked  into  an  elaborate  theory  of  inheritance,  which  was 
published  in  complete  form  in  his  book  on  The  Germ  Plasm 
in  1892.  The  chromosomes  were  regarded  as  aggregates  of 
different  elements,  each  element  (called  a  biophor)  representing 
some  specific  characteristic  or  group  of  characteristics  of  the 
adult  organism.  With  longitudinal  division  of  the  chromo- 
somes, each  element  is  equally  divided. 

Maturation  Phenomena. — In  the  meantime,  the  discovery  had 
been  made  that  the  mature  germ  cells,  when  ready  for  fertiliza- 
tion, contain  only  half  the  number  of  chromosomes  character- 
istic of  the  species,  so  that  upon  union  of  egg  and  spermatozoon 
the  normal  number  is  restored,  the  resulting  offspring  inheriting 
equally  from  the  two  parents.  It  had  also  been  found  that  this 
halving  in  number  of  chromosomes  takes  place  in  the  germinal 
endothelium  during  the  process  of  ripening  of  eggs  and  sperma- 
tozoa, and  at  the  time  of  the  preliminary  mitoses  which  accom- 
pany the  formation  of  the  germ  cells.  These  peculiar  divisions 
became  known  as  the  maturation  divisions. 

After  the  discovery  of  the  reduced  number  of  chromosomes, 
and  in  accordance  with  his  conception  of  the  chromosomes  as 
the  bearers  of  hereditary  characteristics,  Weismann  in  1888 
prophesied  that,  in  one  of  the  maturation  divisions,  it  would  be 
found  that  the  chromosomes  do  not  divide  longitudinally  but 
transversely,  so  that  the  hereditary  characteristics,  instead  of 
being  equally  partitioned  between  the  daughter  cells,  would  be 
divided  crosswise,  so  that  the  daughter  cells  would  receive  dis- 
similar groups  of  biophors.  The  ordinary  longitudinal  division 
of  the  chromosomes  he  called  an  equation  division,  and  the 


212 


THE  PERPETUATION  OF  ADAPTATIONS 


extraordinary   hypothetical   division   during   maturation,    the 
reduction  division. 

The  fulfilment  of  this  prophecy  by  a  host  of  different  observ- 
ers was  a  remarkable  justification  of  the  imagination  in  science. 


H 


FIG.  89. — Middle  and  end  phases  of  mitosis.  G,  Metaphase  showing  the 
longitudinal  split  of  the  chromosomes;  H,  the  end  phase  or  anaphase  with  the 
daughter  chromosomes  separating,  between  them  the  inter-zonal  fibers  (if); 
the  centrosomes  are  divided  in  preparation  for  the  next  following  mitosis;  /  and 
J,  final  stages  in  daughter  nuclei  formation  and  division  of  the  cell;  n,  the  dis- 
carded nucleolus;  ep,  equatorial  plate.  (From  Wilson.) 

The  reduction  division,  in  some  form  or  other,  often  complicated 
and  atypical,  was  revealed  in  type  after  type  of  animals  and 
plants,  until  today  it  is  generally,  if  not  quite  universally,  ac- 
cepted as  a  typical  phenomenon  of  maturation. 


MATURATION  PHENOMENA 


213 


The  Maturation  Divisions. — The  maturation  divisions  in  male 
and  female  organisms,  while  similar  so  far  as  the  chromatin  is 
concerned,  do  not  result  in  the  formation  of  the  same  number 


PRIMARY 
SPERMATOCYTE 


SECONDARY 
3PERMATOCYTE5 


PRIMORDIAL  GERM-CELLS 


WITH  DIPLOID  NUMBER  OF  CHROMOSOMES  X 
N  THIS  DIAGRAM) 


MULTIPLICATION  PERIOD 

MANY   GENERATI 

SPCKMATO 


GROWTH  PERIOD 
5YNAPSIS 

UNION   OF  CHROMOSOMES   IN  PAIRS 

HAPLOio  NUMBER)  OF  BIVALENTS 


BIVALENTS  LONGITUDINALLY  SPLIT 
FIRST  MATURATION  DIVISION 

I  SPLIT    CHROMOSOMES 

MATURATION  DIVISION 


SINGLE 
CHROMOSOMM 


PRIMARY   OOCYTE 
(OVARIAN  EGG) 


SECONDARY    OOCYTES 

EGG—  1"  POLAR    BODY 


POLAR    BODY 

(w  SOME  CASES)  DIVIDES 

-z-  POLAR  BODY 


FERTILIZATION 


FIR3T  CLEAVAGE 


FIG.  90. — Diagram  of  the  maturation  divisions  of  the  male  and  female  germ 
cells.  Four  chromosomes  are  present  in  all  cells  of  the  body  of  the  case  illus- 
trated. (The  polar  bodies  are  represented  as  much  larger  than  they  actually  are 
in  relation  to  the  egg  cell.) 

of  mature  germ  cells.  From  each  primordial  egg  cell  only  one 
mature  egg  is  formed,  while  three  rudimentary  eggs  called  polar 
bodies  are  formed,  which  have  no  part  in  development,  but  de- 
generate and  die.  From  each  primordial  cell  of  the  spermato- 


214          THE  PERPETUATION  OF  ADAPTATIONS 

zoon,  on  the  other  hand,  four  functional  spermatozoa  are  formed, 
each  of  which  may  fertilize  an  egg.  In  each  case,  the  primor- 
dial germ  cells  of  the  germinal  endothelium  are  similar;  each 
has  the  number  of  chromosomes  characteristic  of  the  species 
(in  modern  terminology,  the  diploid  number),  but  the  egg- 
forming  cells,  at  an  early  period,  begin  to  enlarge  and  to  deposit 
stores  of  yolk  in  the  cell  body.  The  chromatin  of  the  nucleus 
collects  in  a  thick  fibrous  mass  on  one  side  of  the  nucleus 
(synapsis  stage),  and  from  it  emerge  one-half  as  many  chromo- 
somes as  are  formed  at  ordinary  vegetative  divisions  (in  modern 
terminology,  this  is  called  the  haploid  number) .  Each  chromo- 
some, however,  is  double,  consisting  of  two  chromosomes 
lying  side  by  side  or  end  to  end  (Fig.  90) .  Reduction,  therefore, 
at  this  stage  has  not  actually  taken  place,  hence  the  phrase 
pseudo-reduction  is  applied  to  it.  The  twor  parallel  parts  of 
each  chromosome  then  divide  longitudinally,  and  the  entire 
chromosome,  in  many  cases,  contracts  into  a  smaller  four- 
parted  chromosome  termed  a  tetrad  .(Fig.  90).  A  mi  to  tic 
figure  is  then  formed,  which  migrates  toward  the  periphery  of 
the  egg,  and  the  nucleus  divides  equally,  one-half  of  each  tetrad 
passing  into  a  daughter  nucleus.  While  the  nucleus  divides 
thus  equally,  the  egg  cell  divides  unequally;  only  enough  egg 
protoplasm  is  divided  off  to  surround  the  one  daughter  nucleus. 
This  becomes  pinched  off  at  the  surface  of  the  egg  to  form  a 
minute  bud-like  cell  termed  the  polar  body.  Both  nuclei 
then  pass  directly  into  a  second  division  phase,  the  chromo- 
somes undergoing  no  further  change.  By  this  second  divi- 
sion, each  remaining  half  tetrad  (now  called  a  dyad)  is  separated 
into  its  two  component  parts,  one  going  to  each  daughter 
nucleus,  and  a  second  polar  body  is  formed  from  the  egg.  The 
first  polar  body  meanwhile  may  have  divided  to  form  two  small 
cells,  which  with  the  second  polar  body  and  the  functional  egg 
make  up  the  four  cells  derived  from  the  primordial  germ  cell. 
In  many  cases  the  first  polar  body  does  not  divide,  and  in  some 
cases  these  maturation  divisions  do  not  take  place  until  after 
the  spermatozoon  has  entered  the  egg;  sometimes  the  first 
polar  body  is  formed  before,  the  second  polar  body  after, 
entrance  of  the  sperm. 


GERM  CELLS  215 

Reduction  in  number  of  chromosomes  also  occurs  in  the  plant 
world.  Here,  in  many  cases,  the  germ  cells  with  a  reduced 
number  continue  to  proliferate,  and  even  to  give  rise  to  an 
entire  plant  (gametophy te) .  Thus  in  the  fern,  reduction  in  the 
number  of  chromosomes  occurs  at  the  time  of  formation  of  the 
spores  (cf.  p.  122).  Each  spore,  and  all  of  the  cells  of  the  sexual 
generation  formed  from  it  (pro thallium),  thus  have  only  one- 
half  the  number  of  chromosomes  contained  in  the  cells  of  the 
asexual  generation  (sporophy te) ,  the  full  number  being  re- 
stored by  union  of  the  spermatozoid  and  the  oosphere.  While 
details  differ  slightly  in  animals  and  plants,  the  essential 
facts  are  the  same. 

The  male  cells  resemble  the  female  in  the  germinal  tissue,  but 
in  many  types  of  animals  characteristic  changes  soon  appear, 
which  make  these  early  germ  cells  distinctly  different  from 
early  egg  cells.  These  differences  have  to  do  with  the  deter- 
mination of  sex,  which  will  be  considered  in  a  later  section. 
In  other  types  of  animals  no  such  visible  sex-indicating  differ- 
ences appear,  and  in  these  the  nuclear  changes,  formation  of 
tetrads  in  haploid  number,  and  double  division  take  place  as  in 
the  egg.  No  polar  bodies  are  formed,  but  four  functional 
spermatozoa  result  (  Fig.  90). 

The  Germ  Cells  after  Maturation. — If,  on  Weismann's  hypo- 
thesis, the  chromosomes  are  made  up  of  a  series  of  factors 
determining  adult  structures,  they  must  be  variously  distributed 
in  the  germ  cells.  In  Ascaris,  a  nematode  worm,  for  example, 
there  are  four  chromosomes  in  the  early  germ  cells.  We  may 
follow  a  hypothetical  group  of  characters  in  one  of  these  in  sper- 
matogenesis,  as  shown  in  Fig.  91  (upper  series).  The  light  end 
on  one  of  these  four  chromosomes  (A  and  B)  represents  such  a 
group.  In  the  first  maturation  division  (C),  this  group  passes 
undivided  into  one  of  the  daughter  cells  (D),  while  the  other 
daughter  cell  (D')  contains  no  part  of  it.  At  the  second  matura- 
tion division  (D) ,  the  group  is  equally  divided  by  a  longitudinal 
division  of  the  chromosome,  while  D'  divides  into  two  cells 
with  no  part  of  the  group.  Of  the  four  spermatozoa  which 
result,  two  (E,  E)  contain  the  group  of  characters  which  is  not 


216 


THE  PERPETUATION  OF  ADAPTATIONS 


FIG.  91. — Diagram  showing  the  distribution  of  the  chromosomes  in  the  two 
maturation  divisions  of  the  male  (cf)  and  female  (9)  germ  cells  of  the  nematode 
worm  Ascaris.  The  light  spot  at  the  end  of  some  of  the  chromosomes  may 
represent  a  group  of  characteristics  (e.g.,  sexual  characters),  and  its  distribution 
to  spermatozoa  and  mature  eggs  is  shown  in  the  history  of  the  two  divisions. 
(From  Morgan.) 


CHROMOSOMES  AND  SEX 

^ 


217 


a 


||tl9«IM* 


III  ••••••• 

§!••••§••• 

j  A 


/       ? 


FIG.  92. — The  chromosomes  of  the  squash  bug,  Anasa  tristis,  twenty-one  in  the 
male,  twenty-two  in  the  female.  At  maturation  these  chromosomes  pair  two  by 
two,  similar  ones  mating  together  (/),  one  odd  one  (h)  remaining  unpaired  in  the 
male,  but  paired  in  the  female  (/)  and  (H).  After  the  maturation  divisions  of 
each  primordial  germ  cell,  two  of  the  resulting  spermatozoa  will  have  eleven 
chromosomes  and  two  will  have  ten,  while  all  of  the  egg  cells  will  have  eleven. 
If  one  of  the  first  two  fertilizes  the  egg  the  result  will  be  a  female  with  twenty- 
two  chromosomes;  if  one  of  the  latter  two  unites  with  the  egg  the  resulting 
individual  will  be  a  male  with  twenty-one  chromosomes.  (From  Wilson.) 


218          THE  PERPETUATION  OF  ADAPTATIONS 

represented  in  the  other  two  (E',  E')  (Fig.  91,  A-E).  Fertili- 
zation may  be  accomplished  by  either  E  or  E',  and  different 
types  of  adult  will  result.  There  is  a  similar  result  with 
the  maturation  processes  of  the  egg,  where  three  polar  bodies 
and  one  egg  cell  are  formed.  The  eggs  resulting  will  be  dif- 
ferent, according  to  the  disposal  of  the  group  of  characters 
during  maturation. 

Weismann  finds  in  these  phenomena  a  possibility  of  the  origin 
of  variations  which,  once  originated,  may  be  maintained  or 
exterminated  by  natural  selection. 

The  Significance  of  Pseudo-reduction. — The  significance  of 
pseudo-reduction  has  only  recently  been  made  out.  The 
diploid  number  of  chromosomes  is  reduced  to  the  haploid  num- 
ber by  union  of  the  chromosomes,  two  by  two.  This  union  is 
not  haphazard,  but  takes  place  with  remarkable  order  and  pre- 
cision. It  is  best  shown  in  those  animals  in  which  the  chromo- 
somes are  dissimilar  in  size  and  shape,  as  in  Anasa  tristis  (squash 
bug) ,  where  the  twenty-one  chromosomes  of  the  male  differ  in 
size.  It  has  been  demonstrated  that  these  chromosomes  occur 
in  two  sets  of  chromosomes,  as  shown  in  Fig.  92  (e,f).  When 
union  of  chromosomes  takes  place  at  the  period  of  pseudo- 
reduction,  a, unites  with  a,  b  with  b,  c  with  c,  etc.,  so  that  the 
resultant  tetrads  are  symmetrical. 

These  two  sets  of  chromosomes  represent  the  sum  total  of 
character  factors,  received  from  the  two  parents  at  the  time  of 
fertilization  of  the  egg,  which  developed  into  the  adult  whose 
germ  plasm  we  may  be  studying.  Each  set  of  chromosomes 
contains  all  of  the  factors  necessary  for  the  complete  individual 
(as  shown  by  parthenogenesis,  artificial  or  normal).  Each 
chromosome  represents  certain  structures  of  the  adult  (as 
determined  by  experiment),  and  the  union  at  pseudo-reduction 
of  two  similar  chromosomes  is  thought  to  be  the  union  of  the 
factors  having  to  do  with  the  same  characteristics  of  the  adult, 
one  chromosome  representing  these  characteristics  in  the  female 
parent,  the  other  representing  the  like  character  group  in  the 
male  parent.  Hence  it  follows  from  the  happenings  at  matura- 
tion (Fig.  91),  if  the  light  area  represents  certain  characteristics 


MENDELIAN  INHERITANCE  219 

of  the  female  parent,  these  characteristics  will  be  transmitted 
by  the  spermatozoa  E,  E,  and  not  by  spermatozoa  E'E',  which 
transmit  the  male  parent  equivalent  of  these  characteristics. 
After  fertilization  with  these  spermatozoa,  the  offspring  of  E 
will  inherit  this  set  of  characteristics  from  the  paternal  grand- 
mother, while  those  from  E'  will  inherit  from  the  paternal  grand- 
father, subject  of  course,  in  both  cases,  to  modifications  brought 
into  the  union  by  the  egg  cells,  in  which  similar  maturation 
processes  have  taken  place. 

E.  THE  MENDELIAN  PRINCIPLES  OF  HEREDITY 

The  preceding  account  of  the  cytological  changes  during 
maturation,  and  their  interpretation  on  the  basis  of  Weismann's 
theory  of  the  germ  plasm  indicate,  if  the  theory  is  correct,  that 
the  germ  cells,  when  ready  for  fertilization,  are  pure  in  respect 
to  any  given  characteristic,  i.e.,  they  carry  inheritance  of  that 
characteristic  from  either  the  male  or  the  female  parent  and 
not  from  both. 

The  same  conclusion  was  reached,  entirely  independently  of 
cytology  or  of  Weismann's  hypotheses,  through  experimental 
breeding  of  plants  and  animals,  and  is  embodied  in  the  so-called 
Mendelian  principles  of  heredity.  The  great  field  of  modern 
genetics  is  the  outcome  of  experiments  in  selected  breeding 
first  carried  out  scientifically  in  1865  by  Gregor  Mendel,  a 
Silesian  monk,  in  the  gardens  of  the  monastery  at  Briinn.  The 
results  of  his  experiments  and  the  conclusions  he  drew  from 
them  were  published  in  an  obscure  journal,  where  they  remained 
buried  for  thirty-five  years.  In  the  year  1900,  the  botanists 
de  Vries,  Correns  and  Tschermak,  working  independently,  each 
brought  out  evidence  confirming  Mendel's  conclusions,  and  the 
full  value  of  his  work  was  finally  recognized.  Soon  after, 
Bateson  demonstrated  that  Mendel's  principles  apply  to 
animals  as  well  as  plants. 

A.  HEREDITY  OF  ONE  PAIR  OF  CHARACTERS. — Mendel's 
principles  of  heredity  can  be  best  illustrated  by  a  simple  case 
of  his  own,  involving  only  one  pair  of  characters,  in  which  one 


220 


THE  PERPETUATION  OF  ADAPTATIONS 


of  the  pair  comes  from  the  female  parent,  the  other  from  the 
male.  Mendel  crossed  two  types  of  garden  pea,  one  parent 
plant  producing  yellow  peas,  the  other  green  peas.  A  hybrid 
(Fi  generation)  resulted  from  this  cross,  which  produced  only 
yellow  peas.  These  yellow  peas  produced  plants  which  were 


FIG.  93. — Mendelian  inheritance  resulting  from  the  crossing  of  yellow  and  green 
peas.     (From  Morgan,) 

then  intercrossed  or  self-fertilized,  and  the  resulting  peas  (¥2 
generation)  were  found  to  be  mixed,  the  pods  containing  both 
yellow  and  green  peas  in  the  proportion  of  three  yellow  to  one 
green  (Fig.  93).  These,  in  turn,  were  grown  and  again  self- 
fertilized,  when  it  was  found  that  the  green  peas  produced  only 


MENDELIAN  INHERITANCE  221 

green  peas,  while  the  yellow  peas  were  found  to  be  different; 
some  produced  only  yellow  peas,  while  others  produced  yellow 
and  green  peas,  again  in  the  ratio  of  three  to  one.  The  pure 
yellow  peas  were  found  to  be  just  one-third  of  the  total  number 
of  yellow  peas  produced,  or  one-quarter  of  the  total  number 
of  peas — thus,  i  green:  2  mixed,  i  yellow. 

In  reasoning  out  the  significance  of  his  results,  Mendel  con- 
cluded that  something  is  carried  into  the  germ  cells  which  pro- 
duces color  in  the  seeds.  One  original  parent  contained  a  factor 
for  yellow,  the  other  a  factor  for  green ;  fertilization  of  the  germ 
cells  from  the  two  parents  brought  both  factors  into  the  ovule 


o 


F« 


FIG.  94. — Diagram  to  show  the  segregation  and  re-combination  of  the  factors 
(black  and  white)  in  the  gametes,  and  the  presence  of  both  in  the  hybrid  F'. 
(From  Morgan.) 

which  developed  into  the  hybrid.  Both  factors  do  not  come 
out  when  the  peas  are  formed;  one — recessive — remains  latent, 
the  other — dominant — predominates  over  the  other,  with  the 
result  that  all  peas  are  of  one  color,  in  this  case,  yellow.  The 
principle  of  dominance,  where  two  factors  for  the  same  character 
lie  together  in  the  fertilized  egg,  was  thus  recognized.  The 
same  reasoning  is  applied  in  respect  to  all  other  characteristics 
of  the  adult  organisms. 

MendePs  experiments  were  carried  out  long  before  the  essen- 
tial features  of  maturation  were  known;  nevertheless  he  sug- 
gested that  some  process  must  take  place  during  the  formation 


222 


THE  PERPETUATION  OF  ADAPTATIONS 


of  the  hybrid  germ  cells,  whereby  the  yellow  and  green  factors 
are  separated  from  one  another,  so  as  to  produce  germ  cells 
having  only  one  factor  for  green  or  for  yellow.  This  is  known  as 
Mendel's  principle  of  segregation.  The  hybrid  ovule  containing 
both  factors  is  said  to  be  heterozygous,  and  the  two  factors  for 
the  same  character  (here  color)  are  called  allelomorphs ,  one  of 
which  is  dominant,  the  other  recessive.  If  only  one  type  of 
factors  is  present  in  an  ovule,  it  is  said  to  be  homozygous. 


FIG.  95. — Example  of  Mendelian  inheritance  in  which  the  hybrid  (Fi)  is 
intermediate  between  the  two  parents,  but  showing  segregation  of  the  two  factors 
in  the  germ  cells  giving  in  the  F2  generation  the  proportion  of  i  :  2  :  i.  The  cross 
is  made  between  white  and  red  races  of  Mirabilis  jalapa  (the  "four  o'clock"). 
The  hybrid  (F\)  is  pink,  and  these  when  inbred  give  white,  pink,  and  red  flowers 
in  the  proportion  of  i  :  2  :  i  (F2).  (From  Morgan.) 

Now  if  the  hybrid  plants  are  self-fertilized,  i.e.,  through  union 
of  their  own  germ  cells,  and  if  segregation  of  factors  occurs  dur- 
ing the  formation  of  these  germ  cells  in  both  anthers  and  ovules, 
then  the  following  combinations  may  occur  (Fig.  94).  The 
green-bearing  anther  may  unite  with  a  green-bearing  ovule, 
and  the  result  is  green  (homozygous) ;  or  a  green-bearing  anther 


MENDELIAN  INHERITANCE  223 

may  unite  with  a  yellow-bearing  ovule,  and  the  .result  is  a 
heterozygous  yellow,  since  yellow  is  dominant.  Or  a  yellow- 
bearing  anther  may  unite  with  a  yellow-bearing  ovule,  giving  a 
homozygous  yellow;  or  finally  a  yellow-bearing  anther  may 
unite  with  a  green-bearing  ovule,  giving  a  heterozygous  yellow. 
Thus  there  will  be  three  yellows  to  one  green  or  one  pure 
yellow,  two  heterozygous  yellows  and  one  pure  green  (Fig.  94, 
F2  generation). 

Mendel  found  further,  that  these  pure  greens,  if  continually 
self-fertilized,  never  gave  rise  to  yellow  peas,  and  that  the  pure 


PARENTS 


O"     O      © 


FIG.^  96. — Diagram  to  illustrate  the  history  of  the  gametes  of  crossed  white  and 
red  Mirabilis.  _  A  gamete  with  factor  for  white  and  one  with  factor  for  red  unite 
to  form  the  pink  zygote  of  Fi.  The  gametes  in  F\  are  homozygous  for  red  or 
white,  and  these,  by  random  mating,  give  the  Mendelian  ratio.  (From  Morgan.). 

yellows  never  gave  rise  to  green  peas,  while  the  mixed  yellows 
and  greens,  on  self-fertilization,  always  produced  offspring  in 
the  proportion  of  three  yellow  to  one  green. 

A  similar  result  is  obtained  with  white  and  red  races  of 
Mirabilis  jalapa,  the  "four  o'clock,"  in  which  the  hybrid  (F),  is 
pink  (Figs.  95  and  96) .  Here  both  allelomorphs  for  color  take 
part  in  the  hybrid  flower,  forming  a  composite  pink  (Fi)  which, 


224          THE  PERPETUATION  OF  ADAPTATIONS 

on  self-fertilization,  produces  pure  whites,  pure  reds,  and  the 
composite  pinks. 

The  same  result  may  be  worked  out  theoretically  along  the 
lines  of  Weismann's  hypothesis  of  the  significance  of  maturation. 
The  chromosomes  which  unite  at  pseudo-reduction  contain 
allelomorphs  which  are  separated  from  one  another  during  the 
maturation  divisions  (Fig.  90).  Two  kinds  of  spermatozoa 
and  two  kinds  of  eggs  result.  On  self-fertilization,  one  kind  of 
sperm  may  unite  with  an  egg  containing  its  like  kind,  or  it  may 
unite  with  an  egg  containing  its  allelomorph.  Or  the  other 
sperm  may  unite  with  its  allelomorph  or  its  like,  the  result  being 
the  Mendelian  proportion  of  three  to  one. 

B.  HEREDITY  OF  Two  PAIRS  OF  CHARACTERS. — Mendel 
worked  out  the  principles  of  heredity  in  cases  where  two  or 
more  pairs  of  characters  are  involved,  and  found  the  same  under- 
lying principles  of  dominance  and  segregation  as  in  the  case  of  a 
single  pair.  He  crossed  a  pea  producing  yellow  and  round 
seeds,  with  one  producing  green  and  wrinkled  seeds.  The  Fi 
generation  or  hybrid  seeds  were  yellow  and  round,  showing 
that  the  round  characteristic  is  dominant  over  the  wrinkled. 
The  Fi  plants,  when  self-fertilized,  produced  some  yellow  and 
round  peas,  some  yellow  and  wrinkled,  some  green  and  round 
peas,  and  some  green  and  wrinkled  in  the  proportion  of  9  : 3  13:1. 
The  explanation  is  the  same  as  for  the  simpler  cases  of  one  pair 
of  characters.  One  parent  produced  germ  cells  containing  the 
factors  for  yellow  (Y)  and  round  (R) ;  the  other  parent  produced 
germ  cells  containing  the  factors  green  (G)  and  wrinkled  (W). 
The  fertilized  eggs,  or  Fi  generation,  must  therefore  have  con- 
tained YRGW,  the  allelomorphs  being  YG  and  RW.  These 
allelomorphs  are  separated  during  maturation,  the  germ  cells 
containing  either  YR,  YW,  GR  or  GW,  since  these  are  the  only 
possible  combinations.  If  the  hybrids  are  self-fertilized,  there 
would  be  four  kinds  of  male  and  four  similar  kinds  of  female 
gametes,  which  would  give  sixteen  possible  combinations  as 
shown  in  Fig.  97. 

These  experiments  have  been  so  often  repeated,  and  on  so 
many  different  plants  and  animals  with  many  different  charac- 


MENDELIAN  INHERITANCE 


225 


teristics,  that  the  main  conclusions  of  Mendel  are  now  univer- 
sally accepted.  Many  characters,  however,  do  not  seem  to  seg- 
regate or  Mendelize,  at  least  not  in  any  simple  way  that 
can  be  predicted,  and  these  are  the  problems  that  modern 
experimentalists  are  working  on. 


€  O 
CO 
CO 


9 


FIG.  97. — Diagram  illustrating  Mendelian  inheritance  when  two  character- 
istics are  involved.  A  yellow-round  and  a  green-wrinkled  pea  are  crossed.  The 
FI  generation  gives  only  yellow  and  round  peas,  these  being  dominant  over  green 
and  wrinkled.  These  when  inter-bred  segregate  out  in  the  proportion  of 
9:3:3:1.  (From  Morgan.) 

C.  HEREDITY  OF  SEX.  Cytological  evidence. — Sex,  with 
many  of  the  secondary  characters  which  go  with  it,  is  recognized 
today  as  an  aggregate  of  Mendelian  characteristics.  The 
evidence  on  which  this  generalization  is  based  is  partly  cytolog- 
ical,  partly  experimental,  and  while  many  perplexing  problems 
connected  with  sex  are  still  unsolved,  the  evidence  in  favor  of 
the  heredity  of  sex  is  so  strong  that  it  may  be  accepted  as  the 
most  plausible  working  hypothesis  at  the  present  time. 

On  p.  215  reference  was  made  to  divergent  results  in  regard 


226          THE  PERPETUATION  OF  ADAPTATIONS 

to  the  number  of  chromosomes  in  development  of  the  sperm 
cells  of  certain  animals.  In  a  great  many  insects  (belonging 
to  the  orders  hemiptera,  diptera,  homoptera,  phylloxerans, 
etc.),  in  nematode  worms  (Ascaris,  Ancyrocanthus) ,  and  in 
Guinea  pigs,  the  process  of  spermatogenesis  does  not  exactly 
accord  with  the  description  given.  For  example,  the  bug 
Pro  tenor  has  thirteen  chromosomes  in  the  cells  of  the  male,  and 
fourteen  in  those  of  the  female,  instead  of  the  same  number  in 
both  sexes.  Of  the  thirteen  male  chromosomes,  one  is  consider- 
ably larger  than  the  others.  At  synapsis  the  smaller  chromo- 
somes unite  in  pairs  according  to  the  usual  rule,  but  the  large 
one  remains  unpaired  (Fig.  98,  ^  A).  At  the  first  maturation 
division,  all  of  the  chromosomes  divide,  six  small  and  one  large 
passing  into  each  daughter  cell.  At  the  second  maturation  divi- 
sion, the  six  small  ones  divide  again,  while  the  large  one  passes 
undivided  into  one  of  the  daughter  cells  (Fig.  98,  ^  D).  Thus 
two  types  of  spermatozoa  result,  one  type  possessing  six  chro- 
mosomes, the  other,  seven. 

In  the  female  germ  cells  (Fig.  98,  $  A,  B),  there  are  fourteen 
chromosomes,  of  which  twelve  are  smaller  than  the  other  two. 
The  latter  unite  in  synapsis  and  behave  like  the  smaller  chromo- 
somes during  maturation  divisions,  the  resultant  eggs  all  re- 
ceiving seven  chromosomes  (Fig.  98,  9 ,  D,  E,) .  Now  if  a  sperma- 
tozoon with  six  chromosomes  fertilizes  one  of  these  eggs,  the 
result  is  a  male  with  thirteen  chromosomes;  if  one  with  seven 
chromosomes  fertilizes  the  egg,  the  result  is  a  female  with  four- 
teen chromosomes.  The  large,  odd  chromosome,  therefore,  is  a 
sex  determining  chromosome. 

Another  excellent  illustration  is  given  by  the  nematode  worm 
Ancyrocanthus,  where  the  number  of  chromosomes  m£y  be 
counted  in  the  living  germ  cells.  The  male,  as  in  Protenor, 
produces  two  kinds  of  spermatozoa,  one  with  five  chromosomes, 
the  other  with  six.  The  eggs  all  contain  six  chromosomes. 
Fertilization  with  one  type  of  spermatozoa  produces  a  pnale 
with  eleven  chromosomes ;  fertilization  with  the  othert^^  pro- 
duces a  female  organism  with  twelve. 

In  man,  there  is  some  evidence  that  a  similar  difference  in 


INHERITANCE  OF  SEX 
ProTenor  <? 


227 


FIG.  98. — Diagram  illustrating  the  history  of  the  sex  chromosome  X  in  the  bug 
Protenor.  In  the  male  (c?)  there  is  one  large,  unpaired  chromosome  X  (repre- 
sented as  blank)  in  A-Er.  B,  Union  of  the  chromosomes  in  pairs  except  the  X- 
chromosome.  The  two  successive  maturation  divisions  (C,  D),  show  the  dis- 
tribution of  X  in  the  resulting  spermatozoa  (E,  £')>  two  of  the  four  having  X,  two 
having  no  X.  The  latter  have  only  five  chromosomes,  and  on  uniting  with  eggs, 
produce  only  male  individuals.  The  female  organisms  have  two  X's  which  pair 
in  maturation  like  the  other  chromosomes,  so  that  the  resulting  eggs  and  polar 
bodies  all  have  one  X.  A  spermatozoon  with  X,  uniting  with  an  egg,  produces  a 
female  organism.  (From  Morgan.) 


228         THE  PERPETUATION  OF  ADAPTATIONS 

spermatozoa  is  present,  although  the  small  size  of  the  chromo- 
somes and  their  large  number  makes  counting  difficult,  so  that 
observers  disagree  as  to  the  facts.  According  to  one  careful 
observer  (von  Winiwarter),  the  male  cells  contain  forty-seven 
chromosomes  which  unite  to  form  twenty-three  pairs  and  one 
odd  chromosome  (Fig.  99).  Two  types  of  spermatozoa  result, 
one  with  twenty-four,  the  other  with  twenty- three  chromosomes. 


sSfeoc    tteter/ni /Htticn    in   flhin 


•I 


/  * "/  ' 
•  \\ft»t 

U*»#j 


s^x 

B 


FIG.  99. — Diagram  of  the  history  of  the  male  cells  in  human  spermatogenesis. 
A,  Spermatogonium  with  forty-seven  chromosomes;  B,  first  spermatocyte  with 
the  haploid  number  of  chromosomes  in  pairs  and  the  sex  chromosome  (open 
circle);  C,  first  maturation  division;  D,  two  resulting  cells  (spermatocytes)  from 
the  first  maturation  division;  E,  division  of  the  second  spermatocytes  giving 
F,  four  resulting  spermatozoa,  two  female  producing  (above),  two  male  producing 
(below).  (From  Morgan.) 

Female  cells  have  forty-eight  chromosomes,  according  to  this 
observer's  best  counts,  twenty-four  being  present  in  the  mature 
egg.  Fertilization  results  in  an  embryonic  cell  with  forty-eight 
or  forty-seven  chromosomes,  according  to  the  type  of  sperma- 
tozoon uniting  with  the  egg  cell,  and  the  resultant  individual  is 
female  or  male,  according  to  the  type  of  spermatozoon. 

The  cytological  evidence,  therefore,  affords  some  very  clear 
proofs  that,  in  some  cases  at  least,  sex  varies  with  the  presence  or 
absence  of  one  chromosome,  and  we  cannot  get  away  from  the 
conclusion  that,  in  such  cases,  this  chromosome  itself  is  the 


INHERITANCE  OF  SEX 
c? 


229 


>6> 


FIG.  100. — Diagram  illustrating  the  history  of  the  sex  chromosome  when 
accompanied  by  a  smaller  x  or  Y  chromosome,  as  in  Lygaeus  bicrucis.  The  male 
cells  have  twelve  ordinary  chromosomes  and  two  sex  chromosomes,  one  larger 
(X)  than  the  other  (F).  The  resulting  spermatozoa  from  each  primordial  cell 
differ ;  two  have  X  and  produce  females  on  uniting  with  eggs,  two  have  Y  but  no 
X  and  produce  males.  (From  Morgan.) 


230          THE  PERPETUATION  OF  ADAPTATIONS 

essential  factor  in  sex  determination.  In  other  cases,  the  evi- 
dence is  equally  clear,  but  it  is  found  that  the  sex  chromosome  is 
not  always  alone,  i.e.  unpaired  at  maturation.  In  some  cases, 
its  history  may  be  easily  followed  because  of  some  size  difference 
or  other  peculiarity.  Thus  in  the  bug  Lygaeus,  the  sex  chromo- 
some of  the  male  (X) ,  unites  at  synapsis  with  a  smaller  element 
(Y),  the  end  stages  of  maturation  giving  again  two  types  of 
spermatozoa,  one  type  containing  the  sex  chromosome  (X) ,  the 
other  its  smaller  mate  (Y)  (Fig.  100).  The  female,  on  the  other 
hand,  contains  the  full  number  of  chromosomes  including  two 
X's.  After  maturation  there  is  one  X  in  each  egg.  Fertili- 
zation results  in  embryos  with  either  one  X  or  two  X's.  In  the 
former  case,  the  individual  is  a  male,  in  the  latter  case  a  female. 

In  still  other  types  of  bugs,  the  corresponding  X  and  Y 
chromosomes  are  of  equal  size  and  cannot  be  distinguished 
morphologically,  but  males  and  females  are  produced  in  equal 
numbers,  and  the  conclusion  is  justified  that  one  of  the  chromo- 
somes is  the  sex  determining  or  X  chromosome  (see  Ascaris, 
Fig.  91). 

Experimental  Evidence. — The  modern  conception  of  sex  deter- 
mination, as  outlined  above,  is  beautifully  supported  by  direct 
experiments  in  breeding.  It  is  quite  conceivable,  a  priori,  that 
the  sex  chromosome  X  should  contain  factors  standing  for  other 
characteristics  of  the  adult  than  sex  alone.  If  this  is  true,  then 
certain  peculiarities  should  appear  only  when  the  sex  chromo- 
some is  present  as  a  pure  Mendelian  segregation  character. 
An  actual  experiment  will  make  this  clear.  Prof.  Morgan  has 
carried  out  breeding  experiments  on  the  small  fruit  fly,  Droso- 
phila  ampelophila,  for  several  years.  The  wild  fly  has  typical 
red  eyes,  but  during  the  experiments  a  white-eyed  male  ap- 
peared. This  was  mated  to  a  typical  red-eyed  female  (Fig.  101). 
The  offspring  were  all  red-eyed.  These  were  then  in-bred,  and 
the  resulting  brood  contained  (ist)  red-eyed  females,  (2nd) 
red-eyed  males,  and  (3rd)  white-eyed  males,  and  in  the  propor- 
tions of  50%  of  the  ist,  25%  of  the  2nd  and  25%  of  the  3rd,  a 
true  Mendelian  proportion. 

On  the  chromosome  basis,  this  result  is  as  easily  explained 


SEX-LINKED   INHERITANCE 


231 


along  the  lines  of  Mendelian  segregation  as  is  the  case  of  sweet 
peas.  The  sex  chromosome  (Fig.  101)  may  be  represented  by 
X,  the  black  X  representing  red  eyes,  the  open  X  white  eyes, 
and  O  in  males,  no  X.  After  maturation  of  the  original  white- 
eyed  male,  one  type  of  spermatozoa  has  the  open  X,  the  other 
type  has  no  X  as  in  the  case  of  Pro  tenor  (Fig.  98).  Fertilization 
affords  an  equal  chance  for  both  types  of  spermatozoa.  The  egg 


xx 


XXXI 


FIG.  101. — Sex -linked  inheritance  of  white  and  red  eyes  in  Drosophila. 
Parents,  white-eyed  o*  and  red-eyed  9 ;  Fi  red-eyed  <?  and  9  ;  F2,  red-eyed  9 , 
red-eyed  o*  and  white-eyed  c? .  To  right  of  flies  the  history  of  the  sex  chromo- 
somes XX  is  shown.  The  black  X  carries  the  factor  for  red  eyes,  the  open  X 
the  factor  for  white  eyes,  O  stands  for  no  X.  (From  Morgan.) 

cells  contain  one  black  X.  The  Fi  generation  will  all  contain  a 
black  X  which  is  dominant  over  the  open  X,  and  of  course  over 
no  X.  When  these  are  interbred,  two  black  X7s,  a  black  X 
with  an  open  X,  a  black  X  with  no  X,  and  an  open  X  with  no  X 
may  result,  and  the  proportions  are  three  red  to  one  white. 

In  this  experiment,  only  males  appeared  with  white  eyes,  a 
fact  which  might  be  interpreted  as  sex-limited  inheritance,  males 
only  having  white  eyes.  This,  however,  is  not  true,  for  white- 
eyed  females  may  be  produced  by  mating  the  above  red-eyed 


232          THE  PERPETUATION  OF  ADAPTATIONS 

grand-daughters  (containing  a  black  X  and  an  open  X)  with 
the  white-eyed  male s  (containing  an  open  X  and  no  X).  Two 
white  X's  are  brought  together,  and  females  with  white  eyes  are 
produced,  as  well  as  females  with  red  eyes,  and  males  of  both 
types. 

This  feature,  eye  color,  therefore  is  not  sex-limited,  but  is 
said  to  be  sex-linked,  ie.  connected  with  the  sex  chromosome, 
and  is  distributed  with  the  distribution  of  the  sex  chromosome. 

Prof.  Morgan  has  found  no  less  than  seventy-five  of  these  sex- 
linked  factors,  all  of  which  have  been  worked  out  experimentally 
on  the  fruit  fly,  and  all  conform  to  the  case  illustrated  above. 
Other  characteristics  have  been  found  which  have  nothing  to 
do  with  the  sex  chromosomes,  but  are  bound  up  with  others. 
These  results  thus  appear  to  be  a  brilliant  confirmation  of  Weis- 
mann's  hypothesis  of  the  constitution  of  the  germ  plasm. 

F.  THE  ORIGIN  or  VARIATIONS 

The  results  described  above  from  cytological  and  experi- 
mental work  would  seem  to  indicate  that  variations  would  be 
extremely  difficult  to  originate.  If  the  characteristics  of  the 
adult  are  contained  in  the  germ  plasm,  then  the  individual  is 
preordained,  and  the  germ  plasm  would  pass  on  to  descendants 
with  the  same  characteristics.  The  individual  which  develops 
may  change,  by  reason  of  environmental  influences,  in  many 
somatic  characteristics,  but  how  is  it  with  the  germ  plasm  and 
the  characteristics  of  the  parents?  Examples  from  mutilations 
would  seem  to  bear  out  the  Weismann  view,  that  somatic 
changes  of  the  individual  have  no  effect  on  the  germ  plasm 
which  that  individual  carries  and  transmits.  Nevertheless, 
variations  do  arise,  are  transmitted  by  inheritance,,  and  fostered 
or  obliterated  by  natural  selection.  How  they  arise  is  still  a 
matter  of  speculation,  more  or  less  founded  on  fact.  Amphi- 
mixis, mutation,  inheritance  of  acquired  characteristics,  are 
upheld  by  various  biologists  as  accounting  for  the  origin  of 
variations. 

Amphimixis. — The    union    of    germ    cells    brings    together 


THE  ORIGIN  OF  VARIATIONS  233 

(amphimixis)  the  traits  of  two  lines  of  ancestry,  and  with  the 
union  a  possibility  of  different  combinations  in  the  segregation 
of  characteristics.  Or  by  such  union,  recessive  characteristics 
may  be  brought  out,  leading  to  divergent  types  in  the  race. 
This  principle,  advocated  by  Weismann,  leaves  unexplained 
the  origin  of  the  factors  in  the  germ  plasm,  but  interprets  the 
changes  that  may  arise,  as  due  to  shuffling  about  of  the 
characters  already  present.  Other  biologists  interpret  am- 
phimixis as  bringing  about  the  exactly  opposite  result,  viz., 
keeping  the  race  true  to  type,  and  preventing  variations. 

Mutations. — A  number  of  biologists  believe  that  new  types 
arise  suddenly,  by  jumps  or  mutations,  which  first  appear  as 
freaks  of  nature  or  "sports."  The  botanist  de  Vries  discovered 
a  variety  of  primrose  which  underwent  a  spontaneous  change  of 
type,  sufficiently  well  marked  to  make  of  it  a  new  variety,  if  not 
a  new  species.  It  bred  true  to  its  type,  and  showed  no  tendency 
to  revert  to  the  ancestral  form.  De  Vries  concluded,  and  many 
biologists  agree  with  him,  that  freaks  or  sports  appear  infre- 
quently in  the  history  of  every  species,  and  serve  as  centers  of 
departure  from  old  types.  Such  mutations  were  known  to 
Darwin  and  the  earlier  evolutionists,  the  race  of  Ancon  sheep 
being  an  historic  example. 

Mutations  may  be  due  to  the  chance  union  of  recessive  char- 
acteristics, which,  as  in  Prof.  Morgan's  flies,  would  be  lost  again 
by  promiscuous  or  indiscriminate  breeding.  Prof.  Tower  has 
been  able  to  breed  in  the  laboratory  distinct  types  of  the  potato 
beetle,  which  differ  markedly  from  the  ancestral  type  from 
which  they  sprang,  and  he  has  found  the  same  distinct  types 
existing  wild  in  nature  and  regarded  as  different  species.  Here 
an  experiment  was  performed  in  the  laboratory,  which  had 
been  done  on  a  larger  scale  in  nature,  with  the  advantage  in  the 
laboratory,  because  the  starting  point,  a  mutant,  was  known. 
But  again  the  result  may  be  interpreted  as  due  to  shifting  of 
germinal  characteristics,  followed  by  discriminating  breeding. 

If,  in  any  of  these  cases,  the  variation  is  useful  to  the  organism 
in  its  struggle  for  existence,  the  chances  of  living  and  of  mating 
are  increased,  which  would  result  in  the  numerical  increase  of 


234          THE  PERPETUATION  OF  ADAPTATIONS 

the  mutant  type,  until  possibly  the  ancestral  or  original  type  is 
crowded  out.  Thus  by  natural  selection,  a  new  species  and  one 
better  adapted  to  the  environment  would  result.  Or  it  is  con- 
ceivable that  dominance  may  shift  about  with  environmental 
changes,  thus  leading  to  change  of  type.  Speculations  as  to 
the  origin  of  variations  based  on  the  theory  of  mutations  are 
endless,  and  there  is  apparently  good  ground  for  many  of  them. 

The  Inheritance  of  Acquired  Characteristics. — Beginning  with 
Lamarck  (1744-1829),  many  biologists  have  held  that  changes 
brought  about  in  structures  of  the  individual,  during  that  in- 
dividual's lifetime,  are  transmitted  by  inheritance  to  the  off- 
spring. This  conception,  extremely  difficult  to  prove  experi- 
mentally, involves  the  fundamental  principle  of  use  and  disuse 
of  organs  as  affecting  the  descendants  and  the  race. 

Everyone  knows  that  continued  use  of  an  organ  strengthens 
it— a  time-worn  illustration  is  the  blacksmith's  arm — but  there 
is  no  evidence  that  this  over-developed  organ  is  transmitted  to 
the  offspring,  no  evidence  that  the  blacksmith's  children  differ 
from  other  children  in  muscular  development.  This  point  could 
not  be  satisfactorily  proved,  however,  until  a  hundred  or  more 
generations  of  successive  blacksmiths  have  been  studied.  The 
imagination  fails  in  trying  to  account  for  the  origin  of  the  lob- 
ster's chelate  appendages  through  use  and  inheritance,  but 
readily  conceives  how  such  an  organ  might  have  arisen  by  mu- 
tation and  been  transmitted  by  inheritance,  and  which,  being 
useful,  is  preserved  in  the  race  by  natural  selection.  On  the  other 
hand,  the  effects  of  use  seem  to  be  shown  in  the  single-toed  horse 
of  today,  which  has  descended  from  ancestral  forms  having  four 
toes  and  a  rudimentary  fifth  (Eohippus)  on  the  front  legs,  and 
from  forms  having  three  toes,  one  of  which  is  large  and  func- 
tional, the  other  two  reduced  (Hipparion,  Merychippus) .  This 
appears  to  be  a  case  where  continued  use  has  resulted  in  the 
modern  structure. 

The  effect  of  disuse  may  be  readily  imagined.  Vestigial  or- 
gans are  evidence  of  structures  which  hi  the  past  have  been  use- 
ful in  one  way  or  other.  The  lateral  toes  of  the  fossil  horse,  of 
little  use  apparently  even  to  Hipparion,  have  entirely  disap- 


THE  ORIGIN  OF  VARIATIONS  235 

peared  in  the  modern  horse.  The  digestive  tract  of  intestinal 
parasites  shows  various  degrees  of  degeneration  through  disuse. 
In  some  of  the  thread  worms  (nematodes),  it  consists  in  part  of 
a  single  row  of  perforated  cells;  in  the  tape-worm,  it  has  en- 
tirely disappeared. 

In  order  that  they  may  be  perpetuated,  any  changes  that  may 
occur  in  the  individual  must  be  represented  in  the  germ  plasm. 
The  individual  is  mainly  somatic,  and  as  such  is  mortal,  but  he  is 
also  the  nurse  or  protector  of  the  germ  plasm  which  is  potentially 
immortal.  Whether  or  not  he  can  impress  his  individual  stamp 
on  the  germ  plasm,  and  send  it  on  with  the  new  impress,  is  the 
very  crux  of  the  problem  of  species.  Modern  biology  gives  no 
positive  solution  of  this  problem.  Weismann  and  his  school 
maintain  that  all  changes  are  due  to  re-combinations  of  factors 
in  the  germ  plasm;  Neo-Lamarckians,  that  the  germ  plasm 
responds  directly  to  the  changes  in  the  body,  and  these  in  turn 
go  back  to  the  conditions  of  the  external  environment. 

The  limits  of  this  text-book  do  not  permit  an  examination  of 
the  evidence  for  and  against  these  two  points  of  view;  indeed 
many  important  questions  which  bear  upon  it  have  not  been 
even  mentioned.  Morgan  has  recently  summed  up  the  general 
problem  as  follows:  "It  is  true  that  the  germ  plasm  must  some- 
times change — otherwise  there  could  be  no  evolution.  But  the 
evidence  that  the  germ  plasm  responds  directly  to  the  experi- 
ences of  the  body  has  no  substantial  evidence  in  its  support.  I 
know,  of  course,  that  the  whole  Lamarckian  school  rests  its 
argument  on  the  assumption  that  the  germ  plasm  responds  to 
all  profound  changes  in  the  soma;  but  despite  the  very  large 
literature  that  has  grown  up  dealing  with  this  matter,  proof  is 
still  lacking.  And  there  is  abundant  evidence  to  the  contrary. 

"On  the  other  hand  there  is  evidence  to  show  that  the  germ 
plasm  does  sometimes  change  or  is  changed.  Weismann's 
attempt  to  refer  all  such  changes  to  re-combinations  of  internal 
factors  in  the  germ  plasm  itself  has  not  met  with  much  success. 
Admitting  that  new  combinations  may  be  brought  about  in  this 
way,  yet  it  seems  unlikely  that  the  entire  process  of  evolution 
could  have  resulted  by  re-combining  what  already  existed ;  for 


236          THE  PERPETUATION  OF  ADAPTATIONS 

it  would  mean,  if  taken  at  its  face  value,  that  by  re-combination 
of  the  differences  already  present  in  the  first  living  mate- 
rial, all  of  the  higher  animals  and  plants  were  foreordained. 
In  some  way,  therefore,  the  germ  plasm  must  have  changed. 
We  have  then  the  alternatives.  Is  there  some  internal,  initial  or 
driving  impulse  that  has  led  to  the  process  of  evolution?  Or 
has  the  environment  brought  about  changes  in  the  germ  plasm? 
We  can  only  reply  that  the  assumption  of  an  internal  force  puts 
the  problem  beyond  the  field  of  scientific  explanation.  On  the 
other  hand,  there  is  a  small  amount  of  evidence,  very  incomplete 
and  insufficient  at  present,  to  show  that  changes  in  the  environ- 
ment reach  through  thesoma  and  modify  the  germinal  material" 
(T.  H.  Morgan,  Heredity  and  Sex,  pp.  17-18). 

The  origin  of  adaptations  thus,  specifically  in  our  group  of 
Crustacea,  is  still  difficult  to  explain.  Some  advance  of  a  sure 
kind  has  nevertheless  been  made.  We  know  that  a  funda- 
mental property  of  protoplasm  is  its  power  to  vary,  to  adapt 
itself  to  changed  conditions  of  environment.  In  higher  animals 
the  somatic  protoplasm  certainly  exhibits  this  property,  and 
there  is  no  a  priori  reason  why  the  germ  plasm  also  should  not 
possess  it.  We  know  also  that  changes  in  one  organ  bring  about 
compensatory  or  regulating  changes  in  others,  and  again  there 
is  no  a  priori  reason  why  the  germ  plasm  should  not  partake 
in  this  reaction.  An  adaptation  in  our  Crustacea  may  have 
originated  as  a  useful  mutation ;  in  the  germ  plasm,  it  may  have 
been  present  as  a  simple  Mendelian  characteristic,  subject  to  seg- 
regation during  the  maturation  stages.  Later  it  may  have  be- 
come too  deeply  impressed  in  the  germ  plasm  to  undergo  seg- 
regation, and  became  a  fundamental  part  of  the  racial  plasm 
no  longer  subject  to  extinction  by  natural  selection,  while  the 
environment  remained  the  same.  Such  an  origin,  especially  for 
all  of  the  variations  in  a  present-day  phylum,  and  in  different 
phyla,  demands  time.  The  history  of  the  earth,  as  written  in 
modern  geology,  allows  some  hundred  millions  of  years  for 
modern  types  to  have  evolved,  and  if  seventy-five  mutants  of 
a  single  species  may  be  experimentally  produced  in  seven  years, 
it  is  conceivable  that  500,000  species  of  animals  might  have  been 


PRIMORDIAL  PROTOPLASM  237 

produced  in  one  hundred  millions  of  years,  since  each  species 
possesses  the  power  to  vary. 

The  power  to  vary  is  not  possessed  by  all  organisms  in  equal 
degree;  this  is  shown  by  the  very  fact  of  the  enormous  differ- 
ences in  organization  of  modern  animals,  some  of  which  do  not 
vary  in  any  marked  degree  from  forms  deposited  some  fifty 
million  years  ago,  and  found  today  as  fossils.  If  the  dictum 
omne  vivum  ex  vivo  is  true,  then  all  protoplasm  must  be  of 
approximately  the  same  age,  whether  found  in  an  amoeba  or  in 
man.  If  protoplasm  of  all  animals  is  equally  old,  it  follows  that 
some  forms  of  it  were  endowed  with  a  greater  possibility  of 
variations  and  adaptations  than  others,  or  with  a  greater 
"potential  of  evolution,"  so  that  certain  types  developed  into 
marvelously  complicated  organisms  in  the  same  period  re- 
quired by  other  types  to  develop  into  lower  forms.  Thus  if 
man  and  the  coelenterates  are  equally  old,  the  protoplasm  which 
was  to  develop  into  man  must  have  been  endowed  with  a 
much  greater  potential  of  evolution  than  that  which  developed 
into  the  coelenterates,  provided  they  had  a  similar  environment. 

It  is  conceivable  also  that,  in  the  period  when  protoplasm  was 
formed  from  non-living  matter,  the  conditions  were  not  always 
the  same,  and  that  the  "best"  protoplasm,  in  the  sense  of  pos- 
sessing the  highest  potential  of  evolution,  was  formed  during  a 
limited  part  of  the  protoplasm-forming  period,  while  protoplasm 
less  highly  endowed  was  formed  at  other  times,  when  conditions 
were  less  propitious.  Bacteria  and  the  lowest  forms  of  both 
plants  and  animals  might  be  conceived  as  having  come  from 
protoplasm  formed  during  an  unpropitious  period,  and  so  poorly 
endowed  with  the  potential  of  evolution  that  some  of  them 
never  reach  the  stage  of  a  perfect  cell;  others,  like  the  present 
day  infusoria  and  higher  protozoa  generally,  never  progressed 
beyond  the  single-celled  stage. 

If  from  the  coelenterates  up,  a  monophyletic  hypothesis  is 
adequate  to  explain  present-day  phyla,  then  we  must  admit 
that,  with  different  types  and  under  the  conditions  of  their 
development,  unlimited  variations  and  evolution  were  impos- 
sible, and  that  certain  types  of  structure  would  permit  of  many 


238          THE  PERPETUATION  OF  ADAPTATIONS 

more  variations  than  do  other  types.  Some  modifications  were 
capable  of  great  development;  these  were  " lucky  strikes"  so  to 
speak,  in  evolution,  which  enabled  the  organisms  to  respond 
more  quickly  or  more  adequately  to  changes  in  environment. 
One  of  the  greatest  of  these  probably  was  the  development  of 
metameric  structure;  another  was  the  development  of  the  in- 
ternal osseous  skeleton;  still  another  was  the  development  of 
air  tubes  or  tracheae  for  respiration,  and  others  will  occur  to 
every  student. 


GLOSSARY 

— ACQUIRED  CHARACTER.     A  character  which  originates  during  the  life  of 

an  individual  and  due  to  environmental  causes. 

ADOLESCENCE.     Youth,  or  the  period  of  life  between  sexual  maturity  and 
full  development.     Usually  employed  in  connection  with  young  of 
the  human  race. 
-  -ALLELOMORPH.     One  of  two  factors  standing  for  the  same  character  in 

inheritance. 

— ALTERNATION  OF  GENERATIONS.     A  phenomenon  in  the  reproduction  of 

animals  or  plants,  whereby  an  organism  resulting  from  fertilization  or 

parthenogenesis   gives  rise  to  other  organisms    by    some    asexual 

method  of  reproduction  (division,  budding,  or  sporulation). 

AMBOCEPTOR.     An  intermediate  chemical  body  acting  as  the  linking  factor 

between  two  other  chemical  bodies. 

AMINO-ACID.     An  acid  containing  the  amino-group  NH2. 
AMOEBOID.     Pertaining -to  or  resembling  Amoeba. 
— AMPHIMIXIS.     The  union  in  the  fertilized  egg  of  germ  plasm  and  hereditary 

factors  from  different  individuals. 
AMYLASE.     A  ferment  capable  of  dissolving  starch. 
AMYLOLYTIC.     Starch  dissolving. 
— ^/ANABOLISM.     Processes  of  constructive  or  ascending  metabolism,  whereby 

energy  is  absorbed  and  stored  up. 

ANTHEROZOID.     A  minute  male  germ  cell  of  the  fern  and  other  cryptogams. 
ANTIBODY.     A  chemical  substance  capable  of  counteracting  or  neutralizing 

a  toxic  substance. 

ANTIGEN.     Any  substance  capable  of  entering  into  combination  with  proto- 
plasmic molecules  and  of  stimulating  the  formation  of  antibodies. 
APICAL  CELL.     The  single  cell  which  in  higher  cryptogams  constitutes  the 

growing  point. 

—  ARCHEGONIUM.     Female  sexual  organ  of  the  fern  and  higher  cryptogams. 
^ — ARCHENTERON.     Primitive  or  first  gut  of  developing  animal  embryos. 
ARCHESPORIUM.     A  spore-producing  cell. 
AUTOTROPHIC.     Capable  of  independent  or  self-nourishment. 
AXON.     The  main  nerve  process  from  a  nerve  cell;  also  called  the  axis- 
cylinder. 
BASAL  BODY.     Part  of  the  kinetic  complex  of  a  flagellated  protozoan  which 

gives  rise  to  the  flagellum. 

BLASTOMERE.     Any  cell  in  the  early  cleavage  stages  of  the  developing  egg. 
- — BLASTOPHORE.     Sperm  mother-cell  in  earthworm  spermatogenesis. 
— BLASTOPORE.     The  opening  or  mouth  of  a  gastrula. 

239 


240  GLOSSARY 

—  ^BLASTULA.     An  early  stage  in  development  of  the  egg  prior  to  the  gastrula 

or  two-layer  stage. 
BRANCHIOSTEGITE.     Lateral  gill-protecting  portion  of  the  crustacean  exo- 

skeleton. 

yMsuccAL  CAVITY.     Mouth  cavity. 
I/^ARBOHYDRATE.     Any    organic   body    containing   carbon,    with   hydrogen 

and  oxygen  in  the  proportions  represented  by  water  (H2O). 
^*<JELL.     A  unit  mass  of  protoplasm  consisting  of  nucleus  (or  nuclei)  and 

cell  body  or  cytoplasm. 

CENTROLECITHAL.     A  type  of  ovum  in  which  the  yolk  material  is  mainly 
/          collected  in  the  center. 
_     ^/CENTROSOME.     The  center  of  radiations  in  a  dividing  cell. 

CEPHALOTHORAX.     Fused  head  and  thorax  of  the  majority  of  the  higher 

Crustacea. 

CHITIN.     A  lifeless  organic  substance  which  forms  the  basis  of  protective 
membranes,  integuments,  shells,  and  exoskeletons  of  invertebrates. 

—  CHLOROPHYLL.     The  coloring  matter  of  plants  which,  under  the  action  of 

sunlight,    decomposes   carbon   dioxide   and   water   and   recombines 
the  elements  in  the  form  of  carbohydrates. 
CHLOROPLASTID.     A  green,  chlorophyll-bearing  structure  in  plant  or  animal 

cell. 
[/THROMATTN.     The  deeply  staining  substance  of  the  nuclear  network   and 

chromosomes. 

CHROMOGEN.     The  nitrogen-holding  portion  of  the  chlorophyll  molecule. 
CHROMOPLASTID.     A  color-bearing  structure  other  than  chloroplastids   in 

plant  cell. 
~   Vxt^HROMOSOMES.     Deeply  staining  bodies  formed  by  aggregations  of  chroma- 

/        tin  during  the  process  of  indirect  cell  division. 

^CLITELLUM.     A  glandular  swelling  in  the  region  of  the  3Oth  to  37th  somite 
of  the  earthworm.     It  produces  the  girdle  by  which  two  worms  are 
held  together  at  copulation. 
"COELOM.     The  periaxial  body  cavity  of  a  metazoon  with  mesodermal  wall 

and  containing  the  internal  opening  of  the  excretory  organ. 
COLONY.     An  aggregate  or  association  of  individuals. 

COMMENSALISM.     Living   together  in  harmony   without   necessarily   con- 
ferring mutual  benefit  or  harm. 
COMMISSURE.     A  connecting  nerve  between  ganglia. 
COMPLEMENT.     A  chemical  substance  in  the  blood  which  acts  only  through 

association  with  an  intermediate  body  or  amboceptor. 

—  CONJUGATION.     The  temporary  sexual  union  in  protozoa  and  lower  plants. 
CORPORA  LUTEA.     Firm  yellow  bodies  formed  in  the  Graafian  vesicle  after 

the  discharge  of  an  ovum. 

CRUSTACEA.     A  group  of  arthropods  with  firm  exoskeletons. 
I/CUTICLE.     The  lifeless  outermost  covering  of  the  body  of  an  animal. 
CYANOPHYLL.     A  greenish-blue  substance  derived  from  chlorophyll. 
CYCLOSIS.     The  streaming  movements  of  protoplasm  within  the  cell. 
CYSTICERCUS.     The  encysted  state  of  the  larva  of  a  tape-worm. 
The  science  which  deals  with  cells.   • 


GLOSSARY  241 

'YTOPLASM.     The  protoplasm  of  the  cell  apart  from  that  of  the  nucleus;  the 

cell  body. 
— DENDRITES.     The  protoplasmic  branching  processes  of  a  nerve  cell. 

_ DIASTASE.     A  ferment  which  transforms  starch  into  sugar. 

— ^DIFFERENTIATION.     The  evolutionary  process  or  result  by  which  originally 
indifferent  parts  or  organs  become  changed  or  specialized  in  either 
form  or  function;  specialization. 
DIPLOID.     Refers,  in  connection  with  chromosomes,  to  the  double  or  normal 

number,  half  from  the  male,  half  from  the  female  parent. 
DISSEPIMENT.     A  septum  or  partition  between  the  somites  of  annelids. 
-  DOMINANT  CHARACTER.     A  character  inherited  from  one  parent  which  de- 
velops, while  the  factor  for  the  same  character  from  the  other  parent 
remains  latent  or  undeveloped  (recessive). 

ECDYSIS.     Moulting,  or  the  act  of  shedding  an  outer  coat  or  integument. 
ECTOBLAST.     The  outer  primary  cell  layer  in  the  embryo  of  any  metazoan 

animal;  the  ectoderm. 
— ECTODERM.     The  completed  outer  layer  of  cells  in  all  metazoan  animals, 

formed  by  the  cells  of  the  ectoblast. 

ECTOPLASM.     The  outermost  recognizable  living  substance  of  a  cell. 
—  ENCYSTMENT.     The  process  of  forming  a  tough  resistant  covering  or  cyst 

within  which  the  organism  remains  alive. 

— ENDODERM.     The  inner  layer  of  cells  surrounding  the  enteron  in  all  metazoa. 
ENDOENZYME.     A  ferment  formed,  and  normally  acting,  within  the  proto- 
plasm of  a  cell. 

ENDOMIXIS.     Asexual  re-organization  of  the  cell  (Protozoa). 
.ENDOPLASM.     The  inner  protoplasm  of  a  protozoan  cell. 
ENDOPODITE.     The  inner  one  of  the  two  main  divisions  of  the  typical 

limb  of  a  crustacean. 

ENDOTHELIUM.     Superficial  layer  of  cells  derived  from  the  mesoderm. 
ENTERON.     The    intestine,    alimentary    canal,    or    digestive    space    which 

is  primitively  derived  from  the  endoderm. 
.     EPIDERMIS.     The  non-vascular  outer  layer  of  the  body. 
EPISPORE.     The  outer  covering  of  a  spore. 
EPITHELIUM.     Any  superficial  layer  of  cells  of  mucous  membranes  including 

the  proper  secreting  tissues  of  glands,  etc. 
EXOPODITE.     The  outer  one  of  the  two  main  divisions  of  the  typical  limb 

of  a  crustacean. 

FACTOR.     A  specific  cause  in  a  germ  cell  of  a  developed  character. 
/"•FAECES.     Excrement  voided  from  the  anus;  "castings." 
FERMENT.     A  chemical  substance  which  stimulates  chemical  activity  in 

other  substances. 
FERMENTATION.     A  chemical  change  produced  in  an  organic  substance  by 

the  activity  of  ferments  usually  derived  from  living  things. 
GAMETE.     A  reproductive  germ  cell,  male  or  female. 
GAMETOPHYTE.     The  sexual  generation  of  a  plant. 

3LION.     An  aggregate  of  nerve  cells,  nerve  fibers,  and  supporting  cells. 
GASTRULA.     A  stage  in  development  in  which  the  embryo  consists  of  two 
germ  layers  enclosing  the  archenteron. 


242  GLOSSARY 

- — -GASTRULATION.     The  process  of  gastrula  formation. 
GEMMATION.     Asexual  reproduction  by  budding. 
GENETICS.    The  science  of  heredity. 

-GERM  PLASM.     The  reproductive  protoplasm  distinguishe'd  from  the  so- 
matic or  organ-forming  protoplasm  of  the  individual. 
GONAD.     A  reproductive  organ  in  which  the  germ  cells  are  formed. 
HAEMATOCHROME.     A  red  coloring  matter  formed  from  chlorophyll. 
HAEMOCOEL.     A  body  cavity  containing  blood,  and  different  from  a  coelom. 
HAPLOID.     Refers,  in  connection  with  chromosomes,  to  the  half  number 

subsequent  to  reduction. 

HEPATO-PANCREAS.     The  digestive  gland  of  the  Crustacea. 
" — HEREDITY.     The  appearance  in  offspring  of  characters,  the  factors  for  which 

.r  are  in  the  germ  cells. 

^HERMAPHRODITE.     An  organism  with  both  male  and  female  organs  of  re- 
production, or  capable  of  producing  both  eggs  and  spermatozoa. 
^_    _  HETEROZYGOUS.     Containing  two  factors,  or  allelomorphs  for  the   same 

character  in  heredity. 
HOLOBLASTIC.     Cleavage  in  which  the  division  planes  cut  through  the  entire 

cell  mass. 
HOLOPHYTIC.     Like  green  plants  in  the  manufacture  of  food. 

.HOLOZOIC.     Animal-like  in  mode  of  nutrition. 

HOMOLOGY.     Genetic  relation  of  parts;  implies  morphological  likeness  or 
structural  affinity. 

— .HOMOZYGOUS.     In  heredity,  containing  one  kind  only,  of  two  alternative 

factors  for  the  same  character. 
___HORMONE.     An  internal  secretion  necessary  for  the  full  activity  of  some 

organ  at  a  distance. 

HYDRANTH.    A  single  asexual  individual  of  a  hydroid  colony. 

— HYDROID.     Hydra-like,  or  pertaining  to  Hydroidea,  a  group  of  coelenterates. 

HYDROLYSIS.     A  form  of  chemical  decomposition  by  which  a  compound  is 

resolved  into  other  compounds  by  taking  up  the  elements  of  water. 

HYDROLYTIC.     Capable  of  producing  dissolution  through  the  addition  of 

water. 

IMMUNITY.     Protection  against  disease. 
INDUSIUM.     Membrane  covering  a  sorus  or  fruit-dot  in  ferns. 

INTUSSUSCEPTION.     Reception  of  foreign  matter  by  all  parts  at  once  of 

living  matter,  leading  to  interstitial  growth  as  opposed  to  growth  by 
^y          accretion  or  addition  on  the  outside. 
— IRRITABILITY.     The  property  possessed  by  all  protoplasm  of  responding 

to  stimuli. 

— KARYOKINESIS.     The  phenomena  of  nuclear  division  involving  the  forma- 
tion and  the  division  of  chromosomes;  same  as  mitosis. 

..J/KATABOLISM.     Destructive   metabolic    processes   in   the   living    organism, 

whereby  protoplasm  and  its  derivatives  are  broken  down,  forming 

compounds  of  lower  energy  potential  and  transforming  stored  energy 

into  energy  of  heat  and  movement. 

LININ.     The  substance  of  the  nuclear  reticulum  other  than  the  chromatin. 


GLOSSARY  243 

, -LIPASE.     An  enzyme  which  converts  fats  into  glycerine  and  fatty  acids. 

LIPOLYTIC.     Capable  of  disintegrating  fats. 
MACROCYTASE.     Digestive  ferment  of  the  macrophage. 

MACRONUCLEUS.    The  larger  nucleus  of  a  protozoan  cell  in  which  dimorphic 

nuclei  are  present. 
— 'MATURATION.     The  series  of  processes  in  the  formation  of  germ  cells  by 

which  the  number  of  chromosomes  is  reduced  to  one-half. 
MEDUSA.     A  free-swimming,   gonad-bearing  sexual  generation  of  coelen- 

terates. 

MELANIN.     A  toxic  pigment  formed  by  malaria  organisms. 
MERISTEM.     Unformed  and  growing  cell  tissue  found  at  the  ends  of  young 

stems,  leaves  and  roots. 
MEROBLASTIC.     Applied  to  eggs  in  which  the  division  or  cleavage  planes  do 

not  cut  through  the  yolk  mass;  superficial  cleavage. 
MEROZOITE.     An  asexually  reproduced  germ-cell. 
— MESODERM.     The  middle  germ  layer  of  an  animal  embryo  in  the  three-layer 

stage. 

(^^METABOLISM.     The   aggregate   of   chemical   changes   in   living   organisms 
involving  the  building  up  of  protoplasm  (anabolism),  and  the  break- 
ing down  of  protoplasm  (katabolism). 
METAGENESIS.     See  alternation  of  generations. 

/METAMERISM.     Segmentation  of  the  body  along  the  main  axis,  resulting  in 
a  series  of  more  or  less  similar  parts  which  are  serially  homologous. 
MICROCYTASE.     A  digestive  ferment  produced  by  microphages. 
.— MICRONUCLEUS.     The  smaller  nucleus  of  an  infusorian  in  which  dimorphic 

nuclei  are  present. 
MICROSOMES.     The  minute  granules  embedded  in  the  ground  substance  of 

protoplasm. 

MIMICRY.     The  simulation  of  something  else  in  form  or  color,  usually 
having  protective  value  to  an  organism. 

The  processes^  involved  in  nuclear  division,  including  formation 

^      and  division  of  t.hft  chromosomes.     Same  as  karyokinesis. 
,— ^MORPHOLOGY.     The  science  which  deals  with  form. 
—  MUTATION.     The  process  of  originating  a  new  species  or  a  new  specific 

character  at  a  single  step;  discontinuous  variation. 

NEMATOBLAST.     A  nettle  thread-forming  cell  of  Hydra  and  allied  forms. 
NEMATODE.     A  round-  or  thread-worm. 
NEPHRIDIUM.     The  excretory  organ  of  invertebrate  animals. 
NEPHROBLAST.     Initial  cell  of  a  chain  of  cells  in  development  destined  to 

form  an  excretory  organ  or  part  thereof. 
NEPHROSTOME.     Mouth  or  internal  opening  of  a  nephridium. 
NEUROBLAST.     Initial  cell  in  a  chain  of  cells  in  development  destined  to  form 

the  nervous  system  or  part  thereof. 

^.^NEURON.     Morphological  and  physiological  unit  of  the  nervous  system  con- 
sisting of  a  nerve  cell,  its  nucleus,  axon,  and  dendrites. 

— XJ\  UCLEUS.     A  differentiated  portion  of  the  cell  protoplasm  consisting  of  mem- 
brane, chromatin,  linin,  nucleoli  and  ground  substance. 


244  GLOSSARY 

OMMATIDIUM.     A  radial  element  or  segment  of  the  compound  eye  of  an  ar- 
thropod. 
ONTOGENY.     The  developmental  history  of  a  given  organism  as  distinguished 

from  phylogeny  or  history  of  the  race. 

OOGENESIS.     The  development  of  the  ovum  from  a  primordial  sex  cell. 
OTOCYST.     A  vesicle  associated  with  the  sense  of  equilibration  in  lower 

animals;  a  primitive  auditory  organ. 

OTOLITH.     A  mineral  element  or  concretion  in  an  auditory  vesicle. 
OXIDATION.     The  action  or  process  of  taking  up  or  combining  with  oxygen. 
PARABASAL  BODY.    A  part  of  the  kinetic  complex  of  a  flagellated  protozoan. 
—PARENCHYMA.     The  fundamental  cellular  tissue  of  plants. 

PARTHENOGENESIS.     Development  without  fertilization  of  an  egg  into  a 

normal  individual. 

PERICARDIUM.     The  membrane  around  the  heart. 
PERISTALSIS.     Wave-like  involuntary  contraction  of  circular  muscles  of  a 

tubular  organ. 

PERISTOME.     The  region  around  the  mouth. 
PHAGOCYTE.     Amoeboid  cell  of    the   blood  able  to  engulf  other  cells  or 

foreign  objects. 

PHAGOCYTOSIS.     The  process  of  engulfing  by  a  phagocyte  or  white  blood  cell. 
-PHOTOSYNTHESIS.     The  process  by  which  green  plants  utilize  the  energy  of 

sunlight  in  the  manufacture  of  starch. 
^PHYLOGENY      That  branch  of  biology  which  treats  of  the  ancestral  history 

of  animals  or  plants. 

PHYLUM.     Any  primary  group  in  the  animal  or  vegetable  kingdom. 
PHYTOL.     A  primary  alcohol  composing  part  of  the  chlorophyll  molecule. 
PINNAE.     The  smaller  branches  of  a  branching  structure. 
PINNULES.     The  smallest  branches  of  a  branching  structure. 
PLASMODIUM      The  cause  of  malaria,  a  protozoan. 

-POLAR  BODY.     A  minute  abortive  cell  given  off  by  an  ovum  during  matura- 
tion. 
POLYMORPHISM.     Capacity  of  an  animal  or  plant  to  exist  under  different 

forms  or  types. 
PROCTODAEUM.     The  posterior  part  of  the  digestive  tract  of  an  animal 

formed  by  the  ingrowth  of  ectoderm. 
PROGLOTTID.     One  of  the  posterior  segments  of  a  tape- worm. 

STOMIUM.     The  lobe  in  front  of  or  overhanging  the  mouth  of  an  annelid. 
PROTEASE.     An  enzyme  capable  of  transforming  proteins  into  diffusible 

bodies. 
— "PROTEIN.     That  group  of  chemical  substances  which  consist  essentially  of 

amino  acids  and  their  derivatives. 

PROTEOLYTIC.     Capable  of  breaking  down,  or  dissolving,  protein. 
PROTEOSE      A  secondary  protein  derivative. 
PROTHALLIUM.     Sexual  generation  derived  by  germination  of  the  spore  in 

the  higher  cryptogams  and  bearing  the  sexual  organs. 

PROTONEMA.     Outgrowth  from  the  germinating  spore  in  higher  cryptogams, 
>•  which  develops  into  the  prothallium. 

V.PROTOPLASM.     The  living  substance  of  animals  and  plants. 


GLOSSARY  245 

PROTOPODITE.     The  first  or  basal  division  of  an  appendage  of  a  crustacean. 
PSEUDOPODIUM.     A  temporary  prolongation  or  protrusion  of  the  proto- 
plasm of  amoeboid  cells. 
QUARTAN    MALARIA.     Recurrent   chill   and   fever   on   every   fourth    day. 

Caused  by  Plasmodium  malariae. 
RECEPTOR.     The  molecule  in  protoplasm  with  which  a  toxin  or  various 

metabolic  elements  may  unite. 

— RECESSIVE.     In  heredity,  a  factor  which,  although  present  in  a  hetero- 
zygous individual,  remains  undeveloped. 

REDUCTION.     The  halving  of  the  number  of  chromosomes  in  the  nucleus  of 

a  germ  cell  during  maturation. 
REGIONAL   DIFFERENTIATION.     Specialization  of  a  part  of  the  body  not 

duplicated  in  other  parts. 
RHIZOID.     Resembling  a  root. 
RHIZOME.     An  underground  trunk  or  stem. 

ROTIFER.     Minute  multicellular  animal  with  rings  of  powerful  cilia;  "wheel- 
animalcule." 
SAPROPHYTIC.     Food-taking  by  absorption  or  osmosis;   applies  to  some 

plant  forms. 

SAPROZOIC.     Same,  applying  to  animal  forms. 

SARCODE.     A  term  proposed  by  Dujardin,  replaced  by  term  protoplasm. 
SCHIZOGONY.     The  process  of  asexual  multiplication  in  certain  types  of 

parasitic  protozoa. 

SCOLEX.     The  "head"  or  attaching  segment  of  a  tape-worm. 
- — SEX-LINKED.     Any  character  the  factor  of  which  is  associated  with  the  sex 

determiner. 

SINUS.     A  cavity  or  hollow  in  tissues. 
— SOMATIC  PLASM.     Protoplasm  of  the  body  organs  and  tissues  as  opposed  to 

the  reproductive  or  germinal  plasm. 
SOMATOBLAST.     A  particular  cell  in  early  development  destined  to  give 

rise  to  the  ventral  plate  of  the  embryo. 

SORUS.     One  of  the  aggregates  of  spore  cases  on  the  fronds  of  ferns. 
- -SPERM  ATOGENESIS.     The  development  of  spermatozoa  from  the  primitive 

or  primordial  sex  cells. 

SPERMATOPHORE.     A  special  capsule,  case  or  sheath  containing  spermatozoa. 
SPIRACLE.     An  aperture  for  admitting  air. 
— f£p.iREME.     A  coiled  mass  of  chromatin  in  thread  form  at  the  beginning  of 

nuclear  division. 

SPORANGIUM.     The  case  or  sac  within  which  spores  are  produced. 
— -^.STEAPSIN.     A  fat-transforming  enzyme. 

STEREOME.     The  woody  elements  which  impart  strength  to  vascular  bundles 

and  other  tissues  of  plants. 

{STIMULUS.     Anything  acting  on  living  matter  which  calls  forth  a  response. 
— STOMA.     Mouth;  a  breathing  pore  in  plant  leaves. 

STOMODAEUM.     The  anterior  part  of  the  digestive  tract  formed  by  ingrowth 

of  ectoderm. 
— SYMBIOSIS.     Obligatory  living  together  of  two  organisms  for  mutual  benefit. 


246  GLOSSARY 

*  SYNAPSIS.     The  union  of  maternal  and  paternal  chromosomes  prior  to 

the  maturation  divisions. 
TAXONOMY.     The  science  of  classification. 
TERTIAN   MALARIA.     Chill  and  fever  on  every  third   day.     Caused  by 

Plasmodium  vivax,  a  protozoan  parasite. 
— TETRAD.     Bivalent   chromosomes   which   appear  to  be   4-parted   in  the 

maturation  divisions. 

"ISSUE.     An  aggregate  of  similar  cells  or  cell  products  having  the  same 
function. 

A  poison;  usually  employed  to  indicate  products  of  protein  break- 
down during  the  metabolic  processes. 

TRACHEAE.     As  used  here,  the  air-holding  tubes  of  insects  and  allied  forms. 
TRICHOCYST.     One  of  the  minute  hair-like  bodies  developed  in  the  cortical 

protoplasm  of  an  infusorian. 

— TfcYPSiN.     A  proteolytic  ferment  capable  of  rapidly  digesting  albumins. 
VTYPHLOSOLE.     A  fold  of  the  intestine  of  certain  annelids  and  other  inverte- 
-    brates,  formed  by  the  inturning  of  the  wall  of  the  intestine  along  the 
dorso-median  line  and  projecting  into  the  intestinal  cavity. 

UREA.     The  final  product  of  protein  decomposition  in  the  body,  forming 

the  chief  solid  constituent  of  the  excretory  fluid  of  many  animals. 
VASCULAR  BUNDLE.    An  aggregate  of   woody  fibers,  cellular  ducts,  and  col- 
umnar cells,  found  in  vascular  cryptogams  and  higher  plants. 

..ViT AMINES.     Substances  of  unknown  chemical  composition  necessary  for 

nutrition  of  the  body. 

XANTHOPHYLL.     A  yellow-green  substance  derived  from  cholorophyll. 
ZOOGLOEA.     A  mass  of  bacteria  embedded  in  jelly  of  their  own  secretion. 
-<^- — ZYMASE.     The  enzyme  of  yeast  which  causes  the  breaking  up  of  sugar  into 

alcohol  and  carbon  dioxide,  or  alcoholic  fermentation. 
ZYMOGEN.     Substance  from  which  enzymes  are  formed  by  internal  changes. 


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E.  BUCHNER  AND  M.  HAHN.     Die  Zymasegarung.     Munich,  1903. 

O.  BUTSCHLI.     Studien  liber  d.  erst.  Entwicklungsvorg.  d.  Eizelle,  d.  Zell- 

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CHARLES  DARWIN.     Vegetable  Mould  and  Earthworms.     Edition  1892. 
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INDEX 

Numbers  in  heavy  type  indicate  illustrations 


A 


Abderhalden  and  Heise,  142 
Absorption  cells  of  the  earthworm, 

142 

Acetic  acid  from  alcohol,  38 
Acquired    characteristics    inherited, 

234 

Active  immunity,  198 
Adaptation,  property  of  protoplasm, 

15;  in  lobster's  appendages,  185; 

and  homology,  203 
Adaptations  against  parasites,  195 
Age  and  natural  death,  68 
Albumen  cells  of  Hydra,  89 
Allelomorphs  in  heredity,  222 
Alternation    of   generations   in   hy- 

droids,  100;  in  ferns,  122 
Alveolar     theory     of     protoplasmic 

structure,  17 
Amboceptors,  in  side-chain  theory, 

2OI 

Amino-acid,  40 

Amoeba  proteus,  44-53;  habitat,  45; 
nucleus,  45;  endoplasm  and  ecto- 
plasm, 46;  vacuoles,  46;  move- 
ment, 22,  47;  metabolism,  47; 
food-taking,  48;  digestion,  48, 
reproduction,  52;  encystment,  53 

Amoeboid  movement,  22 

Amphimixis,  232 

Amphioxus,  cleavage  and  gastrula- 
tion,  80 

Amylase,  40 

Anabolism,  constructive  metabo- 
lism, 12 

Analogy  and  homology,  165 

Anasa  tristis,  pseudo-reduction,  218 

Anatomy,  subject  matter,  2 

Ancyrocanthus,  226 

Animal  associations,  193 

Animal  descent,  204 

Animals  and  plants,  66 

Anterior  and  posterior,  18 

Antero-posterior  differentiation,  133 

Antheridia  and  archegonia,  126 

Anti-bodies  in  immunity,  199 

Antigens  and  anti-bodies,  199 

Apical-cell  growth,  112 

Appendages  of  the  lobster,  168 

Arboroid  colony,  77 

Archegonia  and  antheridia,  126 

Archenteron  in  development,  80 


Archesporium,  123 
Arteries  of  the  lobster,  1 76 
Arthrobranchs,  175 
Ascaris,  chromosome-reduction,  215 
Asexual  generation  of  the  fern,  122 
Auditory  organs  of  the  lobster,  1 80 
Axes  of  symmetry,  18 
Axon  and  dendrites,  153 

B 

Bacteria,  food  of  ciliates,  63;  general 

structures  and  functions,  34-37 
Basal-body,  54 

Basis  of  classification  of  animals,  165 
Bateson,  genetics,  219 
Benham,  earthworm  structure,  147 
Berzelius,  catalytic  forces,  39 
Biedermann  and  Moritz,  173 
Bilateral  symmetry,  135 
Binary  fission,  simple  division,  13 
Biological  sciences,  enumeration  and 

scope,  I 

Biology,  subject  matter,  I 
Biophor,  in  germ  plasm,  211 
Bladder  worms,  192 
Blastophore,  158 
Blastopore  in  development,  80 
Blastula  in  metazoon  cleavage,  79 
Blood    vascular    system    of    earth- 
worm, 145;  of  lobster,  173 
Branchiostegites,  166 
Boas  (figure  of  Trichina),  192 
Botany,  subject  matter,  I 
Bourne^ (figure  of  earthworm),  94 
Branchio-cardiac  sinuses,  175 
Brauer  (figure  of  Hydra),  94 
Buccal  cavity  and  pharynx,  139 
Buchano,  alexine,  197 
Buechner,  extraction  of  xymase,  38 
Buetschli,  rejuvenescence  by  conju- 
gation, 72 


Calciferous  glands,    138;    functions, 

140 

Cartilage,  19 

Castings  of  earthworms,  132 
Catenoid  colonies  of  protozoa,  76 
Cell-division  or  mitosis,  209 
Cells,  protoplasmic  units,  17;  struc- 
ture and  history,  26 


249 


250 


INDEX 


Cellulose  in  animals,  66 
Central    nervous    system    of    earth- 
worm, 150 

Centrolecithal  eggs,  182 
Centrosomes  in  mitosis,  210 
Cephalothorax  of  the  lobster,  166 
Chilomonas  paramecium,   53;  struc- 
ture, 54;  nutrition  and  reproduc- 
tion, 56 
Chittenden,  enzymes  in  metabolism, 

41 

Chlorella  vulgaris,  symbiont,  96 
Chlorogogue  cells,  141 ;  function,  145, 

147 

Chlorophyll  of  fern,  117;  of  Euglena, 

58 

Chloroplastids  in  Euglena,  57;  of 
fern,  117 

Chromogen,  120 

Chromosomes,  210 

Chromulina  flavicans,  double  nutri- 
tion, 66 

Cilia,  definition,  23 

Ciliary  movement,  23 

Claparede,  calciferous  glands,  140 

Cleavage  in  metazoa,  79 

Clitellum  of  the  earthworm,  134 

Codosiga  cymosa,  75 

Coelenterata,  permanent  gastrula 
types,  82 

Coelom  of  the  earthworm,  136 

Coelomic  circulation,  146 

Colonies  of  protozoa,  58 

Colony-types  of  protozoa,  76 

Combault,  calciferous  glands,  140 

Commensalism,  193 

Complement,  in  side-chain  theory, 

2OI 

Conformity  to  type,  207 
Conjugation,    in    Paramecium,    72; 

details,  74 

Consciousness  in  lower  metazoa,  93 
Contractile  vacuole  of  Amoeba,  47 
Co-ordinating  cells,  153 
Copromonas  subtilis,  56 
Correns,  genetics,  219 
Crop  and  gizzard  of  the  earthworm, 

140 

Cuticle  of  the  earthworm,  138 
Cyanophyll  and  xanthophyll,  118 
Cycle  of  living  matter  and  energy, 

121 

Cysticercids  of  Taenia,  192 
Cytological  evidence  of  sex,  225 
Cytology,  subject  matter,  3 
Cytoplasm,  distinct  from  nucleus,  28 

D 

Darwin,  calciferous  glands,  140;  evo- 
lution, 205;  Lumbricus  castings, 
132 


de  Bary,  36 
Dendy,  94 
Denton,  14 

Derivatives  of  the  germ  layers,  161 
Dermal   musculature  of  the  earth- 
worm, 148 
Development    and    metamorphosis, 

182 
Development  of  the  fern,  127;  of  the 

earthworm,    158;   of   the   lobster, 

184 

de  Vries,  genetics,  219 
Diapedesis,  196 
Diastase,  40 
Diblastic  and  triploblastic  animals, 

83 
Differentiations     in     animals     and 

plants,  104-105 
Digestive  system  of  the  earthworm, 

138;  of  the  lobster,  172 
Dileptus  gigas,  effects  of  starvation, 

IO 

Diploid  number  of  chromosomes,  214 
Division,  method  of  reproduction,  13 
Division  of  physiological  labor,  28 
Dobell,  56 

Dominant  characters,  221 
Dorsal  pores  of  the  earthworm,  147 
Dorso- ventral  differentiation,  135 
Drosophila  ampelophela,  230 
Dujardin,  use  of  term  sarcode,  26 


Earthworm,    structures    and    func- 
tions, 130-161 
Ecdysis  or  moulting,  184 
Echinodermata,  radial  symmetry,  83 
Ecology,  subject  matter,  3 
Ectoderm,  of  Hydra,  83 
Ehrlich,  theory  of  immunity,  200 
Embryology,  of  Hydra,  96;  subject 

matter  of,  2 

Encystment  in  Amoeba,  53 
Endoderm,     development,     80;     of 

Hydra,  88 
Endbenzymes,    41;    in    destructive 

metabolism,  50 
Endomixis,  71 
Endophragmal  skeleton  of  lobster, 

167 
Endoplasm      and       ectoplasm      of 

Amoeba,  46 
Endopodites  of  lobster  appendages, 

168 

Endothelium,  146 
Enteric  differentiations  in  metazoa, 

92 

Enzyme  action,  39 
Enzymes,  definition,  39 
Eohippus,  fossil  horse,  234 
Epidermis  of  fern  rhizome,  113 


INDEX 


251 


Epithelio-muscle  cells  of  Hydra,  84 

Equation  division,  211 

Erxleben,  cause  of  fermentation,  38 

Euglena,  54 

Euplectella  with  commensal  crus- 
tacea,  193,  194 

Euplotes  patella  in  division,  13 

Evolution,  205 

Excretion  and  respiration  in  Hydra, 
92 

Excretory  system  of  the  earthworm, 
146;  of  the  lobster,  177 

Exopodites  of  lobster  appendages, 
1 68 

Experimental  biology,  206 

External  apertures  of  the  earth- 
worm, 135 

Eyes,  of  the  lobster,  181 


Facultative  parasites,  193 

False  indusium  of  the  fern,  124 

Farre,  179 

Fate  of  absorbed  food  in  earthworm, 
144 

Ferments,  definition,  39;  in  diges- 
tion, 40 

Fermentation,  alcoholic,  37 

Fern,  chromosome  reduction,  215; 
structures  and  functions,  110-129 

Fibrillar  theory  of  protoplasmic 
structure,  17 

Flagella,  definition,  23 

Flagellated  protozoa,  53-59 

Flemming,  mitosis,  209 

Food  of  animals,  103 

Form  as  a  manifestation  of  vitality, 

17 

Formative  cells  of  Hydra,  ectoder- 
mal,  89;  endodermal,  90 


Galton,  germ  plasm,  208 
Gametophyte,  124 
Ganglia  in  metazoa,  93 
Gastric  mill,  172 
Gastric  vacuoles  of  Amoeba,  47 
Gastrula,  stage  in  development,  79 
Gastrulation  in  metazoa,  79 
General  biology,  subject  matter,  4 
Generalized  organisms,  28 
Gemmation,  method  of  reproduction, 

13 

Genetics,  207;  subject  matter,  3 
Germ  cells   after   maturation,    215; 

in  maturation,  213 
Germination  of  fern  spores,  124 
Gerstaecker,  177,  178,  179 
Gills  of  the  lobster.  174 
Gizzard  of  the  earthworm,  138 


Gonium  pectorale,    a    i6-cell   colony, 

59;  in  reproduction,  78 
Gregaloid  colonies  of  protozoa,  76 
Growth,  a  property  of  protoplasm, 

12 

Guard  cells  of  the  fern,  117 


H 


Habits  of  earthworms,  132;  of  the 
lobster,  166 

Haematochrome,  109 

Haemocoel  of  the  lobster,  174 

Haploid  number  of  chromosomes, 
214 

Harrington,  calciferous  glands,  140 

Harvey,  omne  vivum  ex  vivo,  68 

Hazen,  108 

Hedge,  141,  143 

Hensen,  worm  castings,  144 

Hepato-pancreas,  172 

Heredity,  and  Mendelism,  219;  of 
one  pair  of  characters,  219;  of 
sex,  225;  of  two  pairs  of  charac- 
ters, 224 

Herrick,  183,  185 

Hertwig,  R.,  188,  192 

Heterozygous  germ  cells,  222 

Hipparion,  fossil  horse,  234 

Histology,  of  Hydra,  83;  of  Pteris, 
112;  subject  matter,  3 

Hofer,  experiments  with  amoeba,  49 

Hofmeister,  127 

Holoblastic  cleavage,  79 

Holophytic  nutrition,  55 

Holozoic  nutrition,  55 

Homology  and  classification,  162;  in 
insects,  188 

Homozygous  germ  cells,  222 

Howes,  48 

Homarus  Americanus,  the  lobster, 
166 

Homaxonic  forms,  18 

Hooke,  cells,  26 

Hoppe-Seyler,  chemical  composi- 
tion of  protoplasm,  7 

Hormones,  41 

Huxley,  protoplasm,  6;  spontaneous 
generation,  67 

Hydra,  budding,  13 

Hydra  fusca,  14;  and  Hydra  viridis, 
82;  structures  and  functions,  82- 
96 

Hydroids,  98 

Hypnotoxin,  in  the  nettle  cells  of 
hydra,  90 


Immortality  in  protozoa,  70 
Immunity,  197 
Indusium,  124 


252 


INDEX 


Insects,  1 86 

Internal  structures  of  the  earth- 
worm, 136 

Intra-cellular  digestion  in  Hydra,  91 

Invertase  in  yeast,  39 

Irritability  in  amoeba,  51;  in  hydra, 
93;  in  paramecium,  64 


Jordan,  bacteria  multiplication,  35 


K 


Karyokinesis,  209 
Katabolism,  12 
Kent,  77 

Kitasato    and    von    Behring    anti- 
bodies, 199 


Lamarck,  evolution,  206 

Lang,  174,  184 

Laplace,  physical  and  physiological 

combustion,  n 
Latour,  Cagniard  de,  fermentation, 

38 
Lavoisier,  physiological  combustion, 

II 
Leeuwenhoek,  discovery  of  protozoa, 

68;  of  yeast,  37 
Leidy,  48 
Lenhossek.  153 
Lesser  and  Taschenberg,  142 
Leuckart,  191,  192 
Lifeless  matter  in  living  cells,  19 
Lipase,  40 

Living  and  lifeless  matter,  6 
"Lucky  strikes"  in  evolution,  238 
Lumbricus  terrestris,  135;  structures 

and  functions,  131-160 
Lygaeus,  sex  chromosomes,  229,  230 


M 


Macfadyen,    Morris   and    Rowland, 

yeast,  39 
Macrocytase,  200 
Malaria  organisms,  15 
Maltase  enzyme  of  yeast,  39 
Manifestations  of  vitality,  16 
Marshall  and  Hurst,  84,  86 
Mastigophora,  flagellated  protozoa, 

54 

Maturation     divisions,     213;     phe- 
nomena, 211 

Maupas,  immortality,  70 

McGregor,  141,  143 

Mechanism  of  immunity,  199 


Medusae,  sexual  generation  of  hy- 

droids,  98 
Melanin.,  15 

Mendel,  inheritance,  219 
Mendelian    principles    of    heredity, 

219 

Meristem  of  the  fern,.  114 
Meroblastic  cleavage,  182 
Merozoite,  15 
Merychippus,  234 
Mesoderm,  in  development   80 
Metabolism,  property  of  protoplasm, 

9,  12;  of  amoeba,  48 
Metagenesis  in  hydroids,  100 
Metamerism,  133 
Metamorphosis,  182 
Metschnikoff,  theory  of  immunity, 

200;  phagocytosis,  197 
Microcytase,  200 
Micro gro  mia  socialis,  76 
Microhydra,  82 
Milne- Ed  wards,  177 
Mitosis,  209 
Monaxonic  forms,  18 
Morgan,    216,    220,    221,    222,    223, 

225,  227,  228,  229;  genetics,  230; 

heredity,  235 

Morphology,  subject  matter,  i 
Motor  response  in  paramecium,  65 
Movements   of  protoplasm,   20;  ef- 
fects of  temperature  on,  24 
Muscular  contraction,  23 
Muscular  system  of  earthworm,  147; 

of  lobster,  177 
Mysis-stage  of  the  lobster,  184 


X 


Naegeli,  germ  plasm,  208 
Natural  immunity,  198 
Nauplius  larva,  183 
Nematocysts  of  hydra,  86 
Neo-Lamarckians,  206 
Nephridia  of  the  earthworm,  146 
Nephroblasts,   in  development,   159 
Nerve  cells  of  hydra,  86,  87 
Nervous  system,  of  earthworm,  149; 

of  hydra,  93;  of  lobster,  179 
Nettle  cells  of  hydra,  85 
Neuroblasts  in  development,  159 
Neurology,  subject  matter,  3 
Neurons,  155 

Nitella,    stonewort,    moving    proto- 
plasm, 20,  21 

Nitrobacter,  nitrifying  bacillus,  36 
Nitrosomonas,  nitrifying  bacillus,  36 
Nucleus,  distinct  from  cytoplasm,  28 
Nusbaum,  experiments  with  amoeba, 

49 

Nutrition  of  hydra,  90 

Nutritive  muscle  cells  of  hydra,  88 

Nuttall  and  Buchner,  197 


INDEX 


253 


Obelia,  colony  of  hydroids,  98 
Obligatory  parasites,  193 
Oesophagus  of  the  earthworm,  139 
Old  age,  68 

Olfactory  organs  of  the  lobster,  180 
Ommatidia,  compound  eye  units  of 

the  lobster,  181 
Ontogeny,  15 
Open  and  closed  blood  circulation, 

174. 

Opsonms,  197 
Organisms,  of  one  cell,   26,  44;   of 

tissues,  76 

Organs  and  organ  systems,  130 
Organs  of  relation,  155 
Origin  of  variations,  232 
Ostia,  175 

Otoliths  of  the  lobster,  180 
Oxidation  in  metabolism,  n 


Palaeontology,  subject  matter,  3 

Palisade  mesophyll  of  the  fern,  117 

Parabasal  body,  54 

Paramecium  aurelia,  so-called  im- 
mortality, 70;  parthenogenesis,  71 

Paramecium  caudatum,  60—65;  con- 
jugation, 72;  depression,  63;  divi- 
sion, 64;  effects  of  starvation,  9; 
nutrition,  63 

Parasitism,  190 

Parenchyma  cells  of  fern,  113 

Parker,  G.  H.,  180 

Parker  and  Haswell,  176,  181 

Parthenogenesis,  in  paramecium,  71 ; 
method  of  reproduction,  15 

Passive  immunity,  199 

Pasteur,  yeast  studies,  32 

Pasteur's  fluid,  32 

Pathology,  subject  matter,  3 

Peas,  heredity  of  one  pair  of  char- 
acters, 219;  of  two  pairs  of  char- 
acters, 224 

Pepsin,  digestive  ferment,  40 

Perpetuation  of  variations,  203 

Phagocytosis,  in  hydra,  91;  in  man, 
196 

Photosynthesis  in  the  fern,  119 

Phyla,  in  classification,  81;  of  ani- 
mals enumerated,  163 

Physical  and  physiological  combus- 
tion, ii 

Physiological  adaptation,  190 

Physiological  and  physical  combus- 
tion, ii 

Physiological  balance  of  cells,  44 

Physiology,  subject  matter,  i;  of 
bacteria,  36 ;  of  fern,  1 1 8 ;  of  Hydra, 
90;  of  Pleurococcus,  109;  of  the 
digestive  system,  139 


Phytol,  120 

Pinnae  of  the  fern,  116 

Pinnules  of  the  fern,  116 

Plants  constructive,  animals  destruc- 
tive, 67;  the  food  of  animals,  103 

Plasmodium,  15 

Pleodorina,  colony  differentiation,  78 

Pleurobranchs,  175 

Pleurococcus  pluviatilis,  106-108 

Podobranchs,  175 

Polar  bodies,  213 

Polymorphism  in  coelenterata,  98 

Potential  of  evolution,  237 

Potential  of  vitality  and  old  age,  69 

Primary  germ  layers  in  develop- 
ment, 80 

Proctodaeum,  139,  161 

Proglottids  of  tape- worm,  191 

Properties  of  protoplasm,  7 

Protepse,  49 

Proteins,  classification  of,  8 

Protenor,  sex  chromosome,  226,  227 

Proterospongia,  spheroidal  colony,  77 

Prothallium  of  the  fern,  125,  126 

Protohydra,  82 

Protonema  of  fern,  126 

Protophyta,  unicellular  plants,  28 

Protoplasm,  chemical  composition, 
7;  definition,  6;  theories  of  struc- 
ture, 17 

Protoplasmic  movement,  20 

Protopodites  of  lobster  appendages, 
1 68 

Protozoa,  unicellular  animals,  28 

Pseudopodia  of  amoeba,  47 

Pseudo-reduction,  in  maturation, 
214;  significance,  218 

Pteridium  aquilimim,  the  brake,  no; 
structures  and  functions,  110-129 

Ptyalin,  digestive  ferment,  40 

Purkinje,  protoplasm,  6 


R 


Radial  symmetry,  in  Hydra,  82;  in 

echinodermata,  83 
Recessive  characters  in  heredity,  221 
Rectum  of  the  earthworm,  144 
Redi,  spontaneous  generation,  68 
Reduction   division   in   maturation, 

212 

Reduction  in  chromosomes,  214 

Regeneration  in  Hydra,  95 

Regional  differentiation  of  the  earth-, 
worm,  133 

Rejuvenescence  by  conjugation,  72 

Rennin,  in  yeast,  39 

Reproduction  in  amoeba,  52;  in 
bacteria,  35;  in  earthworm,  158; 
in  fern,  122;  in  Hydra,  94;  in 
Paramecium,  64;  in  Pleurococcus, 
1 06;  property  of  protoplasm,  13 


254 


INDEX 


Reproductive  cells  of  Hydra,  88 

Reproductive  system  of  the  earth- 
worm, 155;  of  the  lobster,  181 

Reserves  of  nutriment,  19 

Reticular  theory  of  protoplasm 
structure,  17 

Retzius,  154 

Reymond,  du  Bois,  protoplasm  de- 
fined, 6 

Rhizome  of  the  fern,  112 

Roux,  significance  of  mitosis,  21 1 


Sachs,  127 

Saprophytic  nutrition,  55 
Saprozoic  nutrition,  55 
Sarcode,  original  name  given  to  pro- 
toplasm, 27 
Sars,  204 

Scaphognathlte  of  lobster,  175 
Schizogony,  15 
Schleicher,  karyokinesis,  209 
Schleiden  and  Schwann,  cell  theory, 

26 

Schneider,  K.  C.,  85,  87,  142 
Schultze,  M.,  identity  of  sarcode  and 

protoplasm,  27 
Schwann,  and  Schleiden,  26 
Scolex  of  the  tape- worm,  191 
Secretin,  action  in  digestion,  41 
Sedgwick  and  Wilson,    18,    19,   21, 
24*  30,  31,  32,  36,  45,  48,    107, 
no,  112,  113,  114,  115,  116,  123, 
124,  127,  134,  136,  147,  148,  151, 
152,  159,  160 

Segmentation  cavity  in  cleavage,  79 
Segregation  of  characteristics,  222 
Senescence  in  protozoa,  68 
Sense  organs  of  the  lobster,  179 
Sensory  cells  of  Hydra,  87,  89 
Sensory  system  of  the   earthworm, 

149 

Serial  homology,  167 
Setae  of  the  earthworm,  134 
Sex  determination,  experimental  evi- 
dence, 230;  in  man,  226,  228 
Sex-determining  chromosomes,  226- 

230 

Sex-linked  inheritance,  232 
Sexual  generation,    of  fern,  124;   of 

hydroid,  98 

Side-chain  hypothesis,  200 
Slime  cells  of  Hydra,  89 
Somatic  and  germ  plasm,  208 
Somatoblasts  in  development,  159 
Somites  of  the  earthworm,  133 
Sori  in  ferns,  124 
Source  of  animal  energy,  103 
Spencer,  germ  plasm,  208 
Sphaerella  lacustris,  108,  109 
Sphaeroidal  colonies,  77 


Spongy  mesophyll,  1 1 7 
Spontaneous  generation,  67 
Sporangium  formation  in  the  fern, 

124 

Sporophyte,  122 

Sporulation,    method    of    reproduc- 
tion, 13 

Starch  formation  in  plants,  118-121 
Steapsin,  40 

Stereome  in  the  fern,  114 
Stigma,  "eye"  in  Euglena,  etc.,  58 
Stomach-intestine,     function,      140; 

structure,  138 
Stomata  of  the  fern,  117 
Stomodaeum,  161 
Suminski,  124  4 
Summary  of  coelenterata,  100 
Supporting  lamella  of  Hydra,  90 
Sweet  wort  culture  medium,  32 
Symbiosis  in  Hydra  viridis,  96 
Synapsis  stage  in  maturation,  214 
Synura  uvella,   colony  of  protozoa, 
57,  58 


Tactile  organs  of  the  lobster,  180 
Taenia  solium,  the  tape- worm,  190 
Tape-worm,  190 
Taxonomy,  subject  matter,  3 
Tetrads  in  maturation,  214 
Tissues,  definition,  28 
Tower,  genetics,  233 
Tracheae  of  insects,  189 
Tradescantia,   circulation   of  proto- 
plasm, 21 

Trichina  spiralis,  192 
Trichinosis,  193 
Trichocysts  of  Paramecium,  62 
Trypsin,  digestive  ferment,  40 
Tschermak,  genetics,  219 
Typhlosole,  141 


U 


Urea,  food  for  bacteria,  37;  disposal 
in  Amoeba,  50;  product  of  me- 
tabolism, II 

Uroglena  Americana,  colony  of  pro- 
tozoa, 58 

Use  and  disuse  of  organs,  234 

V 

Ventral  and  dorsal  differentiation, 
18 

Verworn,  definition  of  irritability, 
52;  experiments  with  Amoeba,  49 

Vestigial  organs,  234 

Vitamines,  42 

Voluntary  and  involuntary  move- 
ments, 24 


INDEX 


255 


W 


Weigert,  hyper-regeneration,  201 

Weismann,  heredity,  208;  matura- 
tion significance,  211;  old  age  and 
death,  70 

Wilson,  E.  B.,  14,  27,  210,  212,  217; 
See  Sedgwick. 

Winiwarter,  sex  chromosomes  of 
man,  228 

Woodruff,  so-called  immortality  of 
Paramecium,  71 

Work  done  by  plants  for  animals, 
128 

Wright,  opsonin,  197 


Yeast,  alcohol  and  acetic  acid  forma- 
tion, 38;  culture  media,  31;  endo- 
enzymes,  39;  fermentation,  37; 
reproduction,  30;  spore  formation, 
31;  structure,  29 


Zoogloea,  jelly  secretion  by  bacteria, 
34 

Zoology,  subject  matter,  i 

Zymase,  39 

Zymogen,  basic  substance  of  en- 
zymes, 37 


Vc 
v> 


Calkins, 0.^. 


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